Black Mold Mildew on Deck Stains
We appreciate it when people write in and ask for advice when something goes wrong during their deck staining projects. Today we talk about what makes your deck prone to black mold growth after application and how you can prevent it. We appreciate your input, so feel free to leave a comment below with pictures of your deck stain projects.
- Over-applying deck stain keeps wood tacky for days, allowing dirt and tree pollen to embed and attract black mold.
- Cheap deck stains use low-grade linseed oil that lacks the mildewcides needed to fight black mold effectively.
- Hot and humid climates or areas near water are far more prone to black mold, mildew, and algae growth on decks.
- Oil-based stains contain natural and synthetic oils that can feed mold growth inside the stain itself if mildewcides are insufficient.
- Water-based stains with zinc oxides are less prone to mold, though they can still attract it once the stain begins to fail.
- RAD Guard Mildew and Algae Cleaner and Preventer removes black mildew and green algae while preventing regrowth for one to two years.
Black Mildew and why it Grows in Certain Deck Stains
Moldy Deck
Exterior deck stains are subjected to the elements making them prone to natural contaminants. A deck stain is there to shield the wood from harsh weather and the damage it can cause such as rot and decay. In providing protection sometimes the deck stain itself can be compromised.
Black mold is one of the natural occurrences that can affect wood stains effectiveness and longevity. There are several reasons why black mold can grow on or in a deck stain but here are the most common reasons.
Over Applying Deck Stain
When a deck stain is over-applied to the wood it can affect the curing or drying time of the stain. When this happens the deck stain remains tacky for several days or longer allowing dirt and tree pollen to become embedded. Once these contaminants have intruded the stain they can attract and begin to grow black mold even after the stain finally cures.
Tips: To avoid curing issues by over applying a wood stain, work in small areas or one board at a time. Let the stain soak into the wood for several minutes then use a brush or a stain rag to wipe any excess drips or puddles.
Inexpensive or Cheap Deck Stain
Going with a cheap deck stain may save you money initially but may cost you more in the long run. Cheaper deck stains consist of a cheap grade linseed oil that lacks the mildewcides that help fight black mold.
Tips: Do some research and buy a quality deck stain that is not prone to mold and mildew.
Humid Climates
In some areas, black mold is almost impossible to avoid. Hot and humid climates or areas next to water are always more prone to all sorts of mold, mildew, and algae. In this situation, black mold can grow on top of a deck stain and jeopardize the overall appearance.
Tips: In this case, use a deck stain that fights against these contaminants while increasing the frequency of deck maintenance to keep black mold from becoming a problem.
Oil-Based Stains
Oil-based stains contain both natural and synthetic oils. These oils can feed the growth of mold “in” the stain itself. All stain manufacturers add mildewcides and algaecides to help prevent this. Some stains like TWP have an EPA registered wood preservative ingredient.
Tips: Use an oil-based stain that has a proven track record of not attracting mold and does not turn black in color.
Water-Based Stains
Water-based stains do not feed the growth of mold like an oil but still can attract mold once the stain starts to fail.
Tips: Choosing a water-based decking stain with zinc oxides will minimize mold.
Best Cleaner to Remove Mold/Mildew on Deck Stains
Pro Tip: The RAD Guard will remove the black mildew and green algae stains from your wood and deck stains and will prevent them from returning for 1-2 years: RAD Guard Mildew and Algae Cleaner and Preventer
| Image | Title | Price | Add to cart | Rating |
|---|---|---|---|---|
 | $49.99 | |||
 | $119.99 |
More information Black Mold & Mildew Decks
- How to Clean a Deck Stain From Mold and Mildew
- Mold and Mildew on Wood Decks
- Black Mold and Deck Stains
- How to Remove Mold and Algae From Decks
- Restore-A-Deck Wood Stain Review
Mildew on Decks
Have a question or comment about black mold and your deck? Please ask below.
Scott,
I have an older Garapa deck (Brazilian hardwood like IPE), which has been stained every few years with TWP 100. Yesterday, I lightly wet the deck and performed Step-1 Strip then followed with a Brightener. After each spray application, I agitated the solution with a deck brush, let dwell, then pressure washed to rinse off. The attached pictures were taken the following morning. I still have large areas of black mold present where rain water tends to dwell over the years. When wet, they become very unsightly.
What do you recommend for a deeper root mold kill before applying a new coat of TWP? Should I be using different products?
Thank you.
Not sure if the is mold or old stain still in the wood. Are you pressure washing or just rinsing after using the stripper? You may need to use a little more pressure. To test/kill mold, use 1 part bleach to 4 parts water. Apply, let dewll for 15 minutes, rinse well with water.
I apply 50/50 TWP 110 and 115 (Honey and Pecan). Never had a dark stain on the wood. I will try the bleach. Thanks.
Scott,
The darker spots are definitely not old stain and the bleach treatment helped. When the boards are wet, I do see old stain which did not get stripped off. I should have provided wet images, as well. Looks like I will need a second application of #1 RAD stripper. I probably didn’t get good enough coverage the first time or I had heavier build ups in spots.
I guess deck staining is like painting — a successful finish depends mostly on good prep work. Thanks, again for your review.
Geordon
100% true. It is all about the prep when it comes to finished appearance and over performance as well. Glad you figured it out.
Hi Scott, thank you in advance for your kind help..
We have a large roof with a spruce or jaw wood construction not impregnated in any way. There are glass panels covering the wood blocking evaporation. The condense water creates wash out and dark brown streaks where it floats off.
I am seeking a method to clean the steaks and protect the wood afterwards.
andre
That is deep in the grain water stains. To try and clean it, use a wood or deck cleaner and then a wood brighener after. It will remove some or most but probably not all of the stain since it is deep in the grain. You will not be able to prevent it from happening again. Stains or sealers will not prevent it.
Hello Scott, thanks a lot for your professional view on this. If i cannot prevent the wash out process in the wood – should the beams be coated at all? I want to prevent structural damage. But since we have to accept the patina – would it be better to leave the beams untreated?
The wood will probably not rot from this condensation staining. Up to you if you want to stain or seal it. If you want to “hide” the staining, using a dark brown stain would make it much less noticeable. Maybe even black or charcoal. Look at the TWP Semi-solid colors. Color chart attached.
Picture of TWP Semi-Solid in Charcoal.
Getting ready to stain a patio deck for the first time in way too many years (we just moved into the house in Colorado). Part of the deck is open to the elements and part is under a balcony. We pressure washed it all and the open area did fine. The area under the balcony however has water stains from the pressure washing (less than 24 hours old). This part of the deck always “stains” when it gets water on it. How do you recommend we clear them up before we stain the deck? Thanks.
If the water stains are deep in the wood grain then they will not come out. Try a Wood Brightener to see if it helps. It would be your only option:
https://www.restore-a-deck.com/rad-brightener-only.html
Help – we are having a new Cumaru wood deck installed and I have noticed black spots on various areas (we are located in Ontario, Canada). I tried pressure washing and it didn’t seem to address. It has not been stained. I was under the impression Cumaru was mild / mildew resistant. We are on an island in an environmentally sensitive area so don’t want to have harmful runoff. Any suggestions?
Looks like rust:
https://www.deckstainhelp.com/remove-rust-black-stains-from-deck/
FYI, no wood is mildew resistant.
I have a 20 year old Ipe deck in Portland Maine.
Every spring I have used the Penofin step 1-2-3 strip-clean-brigjten system. I apply a coat of Penofin Bronze label finish, wipe it off and have a beautiful outdoor living space. By the end of our short summer, black mold is back, and by winter most of the finish is gone
I just turned 60 and if I can’t find a better and easier way to do this, I am going to let it gray.
Penofin has major issues with turning black. We stopped using it 20 years ago due to this. Try a water-based stain so the mildew and mold are minimized. Restore A Deck stain is excellent.
https://www.deckstainhelp.com/restore-a-deck-wood-stain-for-exotic-hardwoods-review/
Make sure to strip and brighten for prep.
I have a question about rot. I have an old douglas fir deck that has begun to rot in places. It’s not quite ready to be completely replaced and I would like to extend its life. After replacing rotten boards and joists and stripping/restaining the surface, I’d like to clean up the mold/fungus that is on much of the underside of the deck that has never been stained. How do you recommend doing that? I’ve read borax on other websites but reading your other comments that doesn’t sound effective. Thanks!
The best bet is bleach but be careful as it will get all over you when you apply it.
I have a 5 year old deck that looks to have black mold spots on top and bottom of the deck boards. Deck was sealed with a Behr product and is peeling off. The deck also gets plenty of afternoon sunlight. Can this mold be cleaned? Do have to replace the boards? The boards themselves are strong. The pictures attached are the top of the deck where it is peeling, a piece of the Behr stain that peeled off and the bottom of my deck.
Try a deck cleaner and a wood brightener to remove.
For the Deck Cleaner which brand do you recommended and for the wood brightener which brand do you recommend? The ones you recommend, Can these be purchased at the local hardware store like Home Depot and Lowes? Also looks like the mold is growing under the stain. Any ideas for this?
No quality products will be found at HD or Lowes. We like the Restore A Deck Prep Products.
I have black mold on my deck, which is about three years old and not yet stained (long story…). I am ready to staining it but want to deal with the mold first.
Before staining it can I use wet and forget, wait a few weeks then stain it? It supposedly can take weeks to work. Will staining over any residual mildew kill it or just make it harder to et rid of afterwards?
I know many suggest detergent or vinegar but I really rather not have to scrub he entire deck with a brush if I can avoid it.
Thanks!
Just clean and brighten the wood while pressure washing, it will remove the mildew. You have to prep anyhow.
I live in San Antonio, TX. Last year I installed a patio deck made of treated pine. After waiting until the wood became absorbent, I opened the pours using a rotary sander and then cleaned the surface with water, letting it dry. I applied two light coats of Behr Waterproofing Stain & Sealer. Three months later I noticed a spotty black mold/mildew like invasion of my wood, the part not under the roof but open to the weather. I contacted Behr, they sent me their All-In-One Wood Cleaner which I applied carefully to their instructions. I worked like a stripper and removed part of the original stain and also with scrubbing, the mold. After using, I washed the deck clean several times with a brush to remove all the cleaner. Then re-applied the Behr Waterproofing Stain & Sealer. Two months later we had our first significant rain, and one week after the rain, can’t believe it, the spotty black mold is back. I can’t even tell you the hours I have logged into this. What can I do? Should I paint over the stain?
You should remove all and start over. Send some pictures for prep help.
I am blown away with this fail. My wife & I are 67 and we did the job now twice ourselves. I can’t bring myself to be fool enough to do it a third time with the same product. I’m considering a Sherwin Williams paint over the stain? But, will it adhere? Here’s a photo from January 2021 before I stripped and restained. After one week of heavy rain, this week I am seeing the same thing happening!
You cannot paint over this but you can use a solid stain over it. Pressure wash for prep first. Use one of these brands:
https://www.deckstainhelp.com/what-are-the-best-solid-color-deck-stains/
Dear Deck Stain Guy – -this is a follow up to your kind dialogue about above with me.
I’m pretty desperate and confused. I need to get this right. Yesterday I had a sales rep from Sherwin Williams come here and look at my deck. Here’s what he is telling me to do. #1 Sand the deck to remove the waxi surface to allow #2 application of SuperDeck Stain & Sealer Remover. Then #3 he wants me to apply SuperDeck Revive Deck & Siding Brightener. Then #4 apply their Exterior Oil Based Primer. And then #5 two coats of Porch & Floor Enamel Interior Exterior Acrylic Satin.
Mr. deck guy, I’m 67 and learned NEVER to apply a water based paint over the top of an oil based paint – it will peel! Mr Sherwin Williams swears this is the way. HELP – – I am drowning in advice. I never thought this project would nearly drive me to tears and frustration.
The SW rep is giving you poor advice. Do not prime and use a porch enamel on this, it will peel. Stay away from SW. They can make paints but their out deck coatings are poor.
Really, the best thing to do is to remove all and start over with a proper penetrating stain that can easily be cleaned and recoated as needed. To remove you will need to strip and sand. Brighten once fully off. You may want to hire this part done.
Stain with one of these top stains: https://www.deckstainhelp.com/the-5-best-deck-stain-reviews-and-ratings/
Dear Deck Stain Guy;
When you say “strip and sand” I understand how to sand, but would you please be more specific please about the stripping product? What product should be used/avoided? When you say “brighten”, again would you please explain how that is done and what product used? Thank you. Pardon me for bringing drama into this, but I’ve been abused so much by paint sales people thus far, suppose it’s normal. Your help is deeply appreciated.
Use the Restore A Deck Stripper with both additives and pressure washing to remove as much as you can. Sand the rest off after it dries, then brighten the wood using the Restore A Deck Brightener:
https://www.deckstainhelp.com/restore-a-deck-stain-stripper-booster-thickening-gel-review/
. . .may I also ask, I looked at your link for 7 Best Deck Stains and I need to know 1) with four layers of Behr waterproofing stain & sealer already on the deck ( granted, the second two coats were preceded by a thorough cleaning with a wood cleaner to take out the mold and loose paint before adding the second two coats) – – with this water based product in the wood, Can I apply the TWP pro series semi solid oil based product over the water based product? Yes I will sand and clean to strip as much as possible first, but will this oil based product resist/react to any of the water based product which may have remnants lingering in the wood?
AND may I ask, seeing that my main issue has been fungal/mold growth may/should I add a mildecide additive to the TWP pro series semi solid oil based paint??
No, you cannot apply any semi-transparent or semi-solid stain over what you have. It must be removed first. There is already fungicides in the TWP stains so no need to add more.
Hi, I live in DC so it’s very humid. Our 20+ year old deck suddenly had some black mold coming up, as well as some green growing in areas. My husband sprayed with white vinegar and then power washed about an hour later. Most of the mold came up, of course with much of the finish too. Pics attached. Do we need to sand and restain? And if so do we need to wait until spring as it looks like our hot days are over – it’s pretty gray, humid, and rains a lot in the fall. Any advice most appreciated!! Thank you.
Strip and brighten all in the Spring is the way to fix this. Look at Restore A Deck products for prep.
Hello! The railing on our Ipe deck is sporting tiny black mold spots. The wood was recently finished with Penifin. Is there anything to do at this point? (Stripping, re-staining, etc are not options) Should I just live with having to wipe down the railings once/week?
To fix you would need to remove it and start over with a better brand of stain.
Okey, you’ve given what could happen to your deck after applying silkens, but no answer about how to clean ioff the black mold without affecting your silkens finish. We don’t need alternative kinds of finishes as we paid a hefty price for what we were told was the best.. Now give us the answer we need for our affected silkens finish.
You cannot clean off the mildew or mold and not create some harm or discoloration to the stain. Bleach should kill it but it will also lighten or discolor the stain.
Not sure why you are being rude in your comments as we do not own Sikkens stains.
I was wondering for the black mold on my unstained deck if I could use a solution of bleach and water!
Bleach is bad for the wood so we would not suggest it but yes, it should kill it.
If my deck has mold on it and mildew green stuff and I have to sand it because of white fuzzies cant I just sand the black mold of instead of using a cleaner? Thanks
No, you need to use a cleaner. Brightener after.
Brighter after cleaner or sanding?
And is it ok to wait a week and then stain?
After sanding. Stain within 2 weeks.
We refinished our deck 8 months ago with your products per instructions. Soon after this occurred and we are very disappointed. Is this mold or something else. Please let me know what we can do to correct the problem. Thank you!
What stain brand? It does not look like mold. More like dirt.
Nope, it doesn’t look like dirt! Mine looks the same and deck isn’t affected under our porch cover.
Yours. Used your entire system. Ended with Restore-A-deck prep and stain.Light walnut semi-transparent. We cleaned, stripped, brightened and stained all with your product. It’s not dirt, we have no dirt on it and it is on our entire large deck. It’s like a grayish build up. What could we use to try and clean it to get rid of this problem? We don’t want to start all over.
Thank you.
~
This site is not ran by RAD products. The RAD stain itself cannot turn gray like this so something else is discoloring it. If on top of the stain, try warm water with dish soap and then lightly scrub with a car wash brush.
Who runs this site? I was under the impression that you worked with restore-a – deck. I have tried water and soap with a brush. Other ideas??? Who should I speak with?
That is explained on the very first page and the very first article. RAD Products do not own this site.
As to your issue, it is not RAD stain related but something else causing this discoloration. After looking at it more closely, did you have an older stain on the deck prior that was not fully removed?
There had been another stain, that is why we contacted this site and sent pics. We went through all the steps we were told to do. If it is the other stains fault, and after all we did, somehow it was not totally removed, now what?? Thank you.
If you did not fully remove the previous stain and that is the reason for the darker discoloration under the RAD stain than the only way to fix is to fully strip it all off and remove the older stain fully that was not removed the first time.
How would you do that? Sand, pressure wash? All thoughts, ideas and instructions are very appreciated. We have put so much time and labor into this deck. Thank you,
Strip off what you can and then sand if needed the rest off. Brightener last.
So essentially start all over. Same product? Strip with what? Sand with what kind of sander and what grain paper? Brightener, same product and then stain?
Any other thoughts other than starting all over?
The other possible option is to try and darken it all with another coat of Dark Walnut and see if that makes it look more even. Clean first before doing this. Just one coat of the stain.
I have black mold spores that have begun to emerge on the wood fencing and railings in my back yard. The wood is pressure-treated wood that was stained/sealed a few years ago. I’m about to restain / reseal all of the wood, but want to know whether I should / need to do something about the black mold spores first. A local company wants a lot of money to come out and pressure wash everything with an “environmentally-friendly” chemical that he says will kill all the mold spores, remove the mold and existing stain, after which we’d restain/reseal. He actually recommends not applying stain/seal and just leaving the wood treated with his chemical wash.
Any thoughts? Do I need to so something about the existing mold spores before resealing all the wood?
Thank you.
You need to strip down the current stain and then brighten the wood. This will remove the mildew as well. Once done you can restain.
I have mold on my deck that simply will not come off. It seems to me the only thing to do at this point is sand which is not ideal bc then I have inconsistent looking stain pattern
Post a picture.
Have a 600 sq.ft. Purple Heart deck 6yr old originally coated with Messmers. When needing recoat I pressure washed & coated with Australian Timber Oil {Cabot}. Has created a White prolific fungus which is dissolving deck boards. Sure it caused by stain as I have spare lbr outside {not coated} and it is fine. Any ideas or suggestions ?
Thanks
You will need to strip it all of and then brighten the wood. Can you post a picture or two?
I did both before applying ATO. The fungus is on the bottom of boards & does not affect the treated 2×4’s on which the floating deck is layed.
A deck stain would not cause this. Not sure what would cause this to happen.
I applied my TWP 1500 stain too heavy. It did not have any puddles of stain, but just over all too much that the wood did not absorb. It is still tacky in spots after drying for 1 week! Is there anything I can do or just let it wear off over the next year????
It will dry and cure fully, just give it more time.
I have over a thousand ceramic coated deck screws (not stainless) with scratched heads down to metal and now black spots are forming around them after staining with TWP 100 and then rain four days later. As the oil based stain doesn’t put a protective coating over the screws (thought there would of been a strong enough film that cured over them) is there another way to seal the screw heads so we can clean and brighten and not have the black spots return?
Sorry but no, a stain will not fix this issue nor can you do anything to prevent the rust outside of replacing all the screws.
That was the answer I dreaded, but somewhat expected. Replace with stainless steel screws in case we need to sand again? Also, since we just put down the TWP, should we wait until spring to do the clean brighten and reapply post screw replacement?
Stainless would be better. You may need to strip and brighten so that the brightener can get deep into the wood to help remove the rust spots.
Thanks. Iron stain is a fresh new topic for me. We cleaned and brightened the deck, then it rained within 2 days. So we waited 3 days for it to dry before staining with TWP 101. Three days later it rained. That’s when we noticed the iron stains. Do you think it stained pre-TWP and we just didn’t notice or are the stains above the TWP? Basically, does oil base staining enhance the iron stains?
No, it does not enhance it. Post a picture.
I posted three above. Here’s another. Seems to mellow a bit after drying, but not much
Here are some pics right after a rain. TWP puddles/ blocks the water, but allows the iron, water and tannins to mix and make ink? Boy, if there’s ever a recommendation to sand a cedar deck without stainless screws better have a warning every time about this pitfall. I never saw it mentioned and I researched alot before going with TWP and proceeding with clean and brighten.
This is not an issue with the prep or the stain. It would have happened no matter what products you used.
Exactly. So please do a service and mention that, if you sand a deck (especially cedar) and you don’t have stainless steel screws or nails, there is a great risk that you’ll end up with unsightly stains after all your hard work AND additional work will need to be done replacing all of the screws or nails as well as cleaning and brightening rework. My point is, there seems to be little or casual mention of the iron stain issue. I know deck experts probably aren’t chemists, I sure am not, but if it’s a known issue, spread the word. I want other folks who research before taking on a big project not to be disappointed. Thanks.
Hi from Spain, I have recently laid my IPE deck, I then stained with Cedria Teka oil( Good Spanish brand) as they told me it gave really good results.
After 2 months black mold spots have turned up. I have read your posts and even the bench I did 3 weeks ago and I haven’t stained yet got the spots.
I have tried the bleach method at 50/50 dilution and rinsed off but the black spots remain.
Any advice on how to get rid of the spots?
Once the spots are gone is it reccomendable to apply a fungicide before oiling?
There seem to be no posts here in Spain with such support as you guys.
First and second picture stained deck
Third and fourth unstained bench
Thanks ,
Aaron
It would be best to strip off the coating as it is embedded in the stain or is under the stain. Brightener after. A fungicide may help when you redo but best to ask the Cedria people for help with this.
Lightly sanded a brand new cedar deck in preparation for stain as recommend by stain manufacturer (Cabot Gold). It rained that evening and the following morning black mold/mildew spots appeared everywhere! I have used the Cabot deck wash (twice) and a water bleach solution as well as trying to re-sanded the spots, but they appear to be in the wood! Anyone dealing with this issue and have a solution. Desperate in Wisconsin…
Post a picture. What did you sand with?
I’ve been applying sikkens 089 redwood to my porch every 2 years for 20 yrs with few problems. I stain the main frame, rafters, and cross members for a unique look after a thorough cleaning before application. The porch is screened. In the past, the finish feels dry after about one week. I am a resident of Delaware and this past year has seen an excess of rainfall in the range of 16 inches above normal. Now the porch has a big black mold problem thru out the structure. The surface of the stain is very tacky like it never cured. But then, I did not stain last year either. The surface of the black mold is not tacky. The question is should i pressure wash the structure to remove the mold? And what about the stain tackiness? Or should i , besides burning it all down, take on the enormous task of stripping and sanding it all out. I dread all the overhead work. Any advice would be appreciated except for arson. Thanks
You will have to remove to fix these issues properly. Sorry.
Two years ago, we had a deck and pergola built onto our house. We coated it with a water sealant/protector the following year. Last year (the third year after it was built), it has green slime type stuff on it, and what looks like mold spots. What can we do to make our fairly new deck wood look new again, and keep it that way? Thank you in advance!
Prep with Restore A Deck Kits and then stain with a penetrating wood stain: https://www.deckstainhelp.com/the-5-best-deck-stain-reviews-and-ratings/
Will this then help it stay looking good? How often should we do this? Meaning, yearly, every two years, etc. ?
Yes. Every two years is normal.
We have a large black greasy/sticky area on our deck. It is near a hummingbird feeder. Could there be a connection?
Yes. Feeds mildew growth.
Thank you for your quick response and advice.
We apparently live in a swamp. After prep with R-A-D products and applying TWP 100 stain our PT pine deck is always covered in mold within 6 months. (We did $20K of drainage improvements around the house. The deck is 2-4 feet above grade.) Is there a more mildew-resistant product than TWP 100 and/or a mildewcide that could be added to the (TWP?) stain to increase the resistance?
TWP already contains an EPA registered mildewcide in the stain. A deck stain like the TW will help prevent mildew from growing in the stain but no deck stain will or can stop mildew from forming on top of a deck stain. It is most likely just an issue with your environment that will have to be dealt with annually.
Thanks. Is there a surface spray mildewcide you recommend that could be applied periodically without degrading the TWP 100?
Not that we have tried. Maybe the “Wet and Forget” would help?
We have green mildew on deck from lack of sun. We tried Behr for mold. It didnt work
, it pealed off like paint. Looking for suggestions
Picture please.
See picture below.
Only way to fix this is to power sand it all off.
once the black is apparent on top of new deck board and in newly applied Behr stain, what do I do. Do i have to strip and startover, will this compromise my new deck. No black was apparent before we stained the new deck after waiting several months. Can I stain over with a solid stain, if so will the black cause my deck to rot?
Strip and or sand it all off and start over is the best option.
Brand new 4″ Tigerwood. Not even finished putting it all down yet (with Stainless steel Camo screws), and not yet oiled/sealed. Tiny black spots have started appearing over large areas. In So Cal, the deck is in the sun a good part of the day, not really been wet yet, although squirted the hose underneath a few days ago to rinse off concrete a couple feet below the deck . But the area where it seems to be popping up most isn’t even over that concrete and is raised about 4 feet off the ground. Doesn’t seem to wash off with dish soap and brush or sanding (which I only tried a very little bit on one small spot by hand). I was planning on Screen sanding the deck with lowish grit like 100 or 120 before Ipe Oil. Now what ?????
Post a picture.
Nothing happens when I click on the Camera ICON to try to attach pic
I tried a different browser. Tried to get a pic showing the black specs. They are appearing out of nowhere over entire boards and sections of the deck.
Try a wood brightener and see if that removes the spots.
Any idea why so many would appear so suddenly?
No idea.
Any Idea what would cause these to appear suddenly out of the blue over large sections?
Those spots look more like stains from metal filings. Any chance you were drilling anything metal near the deck and got some filings on it? Tiny black spots would show up overnight if any moisture is present. Sanding with 60/80 grit will take it out.
I hand scrubbed my deck,looked great. But rained for 2 wks. Black spots (small dots) appeared afterwards so I sprayed it down and scrubbed some of the areas. Some places came up easily and when dried are gone some did not. If these spots are mold and I stain over it,what then? I realise this sounds quite stupid but figured the worst is stain will not last in those areas.
Did you try to kill the mold with bleach? Also try a wood brightener in case the spots are not mold but rust spots.
I painfully hand brushed (2) coats of oil based Arborcoat Stain on to a very large deck of dried out, clean, treated lumber. Black mold slowly appears throughout, every 2-3 months. Looks great when power washed, but is there any products that will slow down the infestation?
Not really. Maybe a product called Wet and Forget?
I live on a lake in a very humid southern city in South Carolina. I like the idea of a penetrating oil stain but am concerned about mildew and mold. Specifically,
what is the best stain for me. I last used an Olympic oil stain with a toner. Beautiful for awhile.
Strip and brighten to remove the Olympic using Restore A Deck Stripping kits. Stain with TWO 100 Series. EPA registered mildewcide in it to help prevent mold.
My deck was stained 2 years ago. The deck was power washed then allowed to thoroughly dry during the hot summer. Black Mike has now appeared. I bleached it with a brush and it looked good for 2 months. Now the mold is reappearing. What do I do?
Strip and brighten the wood for prep and try the Defy Extreme stain.
I have a 9yr old Tigerwood deck. Workers sanded it dien and applied Ipe oil based stain. now i have what looks like mold. I kive at 8k ft in CO so very low humidity. What stain would you suggest for Tiger wood? Shoukd I sand it dien, then clean, brightner, restain in spring or now before winter? thank you. Also i dont see how to post a pic, inly youtube video url or 🙁
Wait until Spring. Strip and brighten for prep with Restore A Deck products. Stain with Defy Hardwood Stain.
We put up all new wood on our deck. The floor is pressure treated and the rails are cedar and powder coated aluminum. We stained the cedar Sikkens. I have noticed this black powder (like soot) only on the top rail (arm rest)of deck. If you took your hand or wet towel and wipe the top rail you would see black on your hand/rail. I tested the deck floor (that is pressure treated wood) and nothing. I check the stained cedar that is vertically on the deck and nothing but everything that is horizontal in cedar has this black powder. It had just rained heavily 3 days prior and my deck is facing southern exposure in a 3+ acre lot that is open. I live in upstate NY Washington County. What could it be? I would say some type of mold but our deck is in direct sunlight from about 8am-5pm
Thanks
It is mold or rust. It is on top of the stained sikkens?
I live in the Pacific NW, so have been very careful to use Restore-a-deck parts 1 and 2, with pressure washing in between. Then I let the deck and rails dry for several days , and sanded them all smooth to help incorporate the 4 year old untreated PT deck into the 1 year new untreated cedar addition. It all looked fabulous, and I then stained in sections. The old deck looks quite good, and the new deck and rails also looked spectacular, for about 5 days. Then they developed black small dots all over them ! They were that areas that had dried really well before staining, and 5 days of sunshine after ! I tried scrubbing with alcohol, then soap- nothing helped ! HELP !!! What went wrong, and how can I fix this ????? I could understand easier if the old deck went spotty, but it did not.
Sounds like maybe rust. How did you sand? It is not related to the stain or prepping products.
So I used a deck cleaner, cleaned the deck well looked great. Let the deck dry for couple of days, deck gets a lot of sun. Put a water based deck stain down and it looked good. By fall I noticed black areas showing up again. Do not understand?
Is the mold on top of the stain and does it wipe off with a wet towel?
i used %100 linseed on a deck after reading a "this old house" article. in a year it was blackened badly by mold and some wood was indented from mold eating – not a simple matter of keeping clean. (i didnt clean with mildewcide and unsure if any product lasts or is safe to continaully apply)
i later found in the (same?) "this old house" article: "use a quality linseed product containing mold inhibitors". (i dont believe i've found one yet, no products are forthcoming about it)
Installed new KDAT on existing joists, now trying to decide between the TWP100 vs Defy Extreme. What are the criteria that should make the final decision between the two products. Do not want to wait till Spring and subject the KDAT to the wet/cold cycle before giving it protection.
In Atlanta where we've had a month of rain, and have had the deck covered with heavy tarp and decking has had zero water since applying. I know that Defy Extreme and TWP 100 are the top rated products, but ….
Make sure to prep first with a cleaner and wood brightener. Stain with either will work but the Armstrong Clark may penetrate your new wood a little better.
Resurfacing deck with new super select KDAT. Pressure washed framing to try to rid of mold. Not all was removed as it had penetrated into 2x10s. Deck next to stream and minimal sun throughout day. Should we replace framing, if so what should be done to precent from reoccurring. Or is it unavoidable in certain areas l(south) and synthetic decking would be better
I would suggest bleaching the framing to help remove the mold. Try apply the Defy Extreme Clear to prevent future mold.
we just built new cedar deck in May, live in Chgo suburbs, used oil based B. Moore arborcoat natural sealant, then 1-2 mos. later the deck was full of large dark stains? Used product recommended by JC Light Pint Store, Lo max and bleach to attack the black stains, mold probably. Applied this mixture again just last week, next day, cedar looked clean, looked great dry, but when it rained next day, dark stains then appeared? How to fix in spring after winter passes in Chgo?
Could be rust as that turns black. Fell free to post some pics in our forum area.
what stains have a track record of not attracting mold or turning black on 2 redwood decks that where one get full sun and the other no sun?
Defy Extreme and TWP 100 Series.
I just cleaned off the black mold and mildew with Dawn and a deck brush. It works great! We had 10 days of rain!
I've been reading the posts and it seems that I have mold both on top and I betted in the stain. Being that it's going to be October. Is it best just for me to wait till spring?
Yes and you should strip and brighten for the prep.
I allowed my new redwood cedar to weather for 9 weeks. I am from Minnesota. Today, I hand sanded it and cleaned with ready to use Cabots wood cleaner for new wood. I used a deck scrub brush on the walking surfaces where a bit of plain old dirt existed. On the kick boards designed from tongue and groove cedar there are 3 small areas that are discolored. Mold or mildew I assume that didn't wash off. Should I allow to dry, sand, and repeat the cleaning process in these areas??
It depends on what the discoloration is. It may come off with cleaning or if tannin stains then a wood brightener should fix it. Make sure to brighten all the wood and not just the affected areas.
Clear coated a new cedar deck. Looks like there is mildew forming under the clear coat. We let it dry for several days after cleaning it before coating it. What can be done?
Strip and brighten for prep. Might want to then wait until next spring to stain.
we have dark spots on our new deck made with pressure treated wood. We have not used a water sealer yet such as "Thompsons". How do we clean this off?
Post a picture please in the forum area.
I have a question on a mold issue.
I stained a pergola last yr. Prep was as follows sanded mill glaze off with 80 grit and pressure washed with tsp, bleach and water. Stained 4 days later with everything dry for those days. Sikkens SRD was what I used for stain. The pergola is near a pool, and a lake. 1 year later it has some black mold in places. Do I need to clean and sand and restain if so what product should I use?
You have to remove the Sikkens. Stain stripper and wood brightener after the stripping. What state are you located in?
We just had our deck stained. The person who stained it did not prep the deck before hand. Therefore the stain is over top of dirty and mold wood. We tried to power wash, but it seems to be tearing up the wood. Is there a deck paint like killz or something we could just paint over top of it? What are your suggestions?
Comments please on this! I just got done watching this old house video where after they got done washing the deck they sprayed something called borate on it to prevent mold and rot. It was 5 parts water to 1 part borate. Sprayed it on and let it dry. Is that a good idea ?? I've never heard of borate and wouldn't know where to even get it.
Next on this video they took 1 gallon of oil based stain and added a quart of varnish to it. Wouldn't that change the formula of the stain if you varnish was added?
That is a bad idea to change the formula of a stain and even worse to add varnish to a horizontal surface. Borates are more for preventing insects. It does not help with mold and rot. We have never bothered to use a borate.
Thank you. It all sounded weird to me as I was watching it. Now if I could just get my deck done here .. handyman guys went on a 2 weeks vacation and are not done yet replacing boards. Then with all the rain we have here in Ohio looks like I will be staining in the fall.
Greetings,
I just received delivery of new redwood (all clear heartwood) and was surprised that
nearly every board has marks on it. It could be mold, but is not dotted throughout the board but instead has marks, some larger but some smaller, sometimes lines that go clear across the 5 1/2 width, perhaps 3-10 different places on each each board. The same type of black marks on at least one board appear to be where the straps were, but they are not all strap marks Before seeing this, I was planning on waiting 2-3 months, then cleaning, then staining. Should I alter my plan and clean now to make sure it cleans up before installing, and, what is the best way to clean in when I do proceed. Thank you, I really appreciate your site! Doug
Not sure if these marks can me cleaned off or if they will need to be sanded off. Either way it should be done now then allow the wood to weather for the 2-3 months, clean, then stain.
Is there any product that will kill the mold on my oiled deck without stripping or damaging the colour?
Probably not. Have you tried soapy water and a light agitation with a very soft bristle brush?
Does a dry below system under the deck cause mold to grow on top deck
Sorry but not sure on this one. I would assume it would not.
Hi,
I just finished building a western red cedar deck and, not surprisingly, as I now learn, it has started to get grey/black stains which arrived after the first rain for several weeks. After a chat with Armstrong Clark I had planned to wait until spring then brighten the wood with oxalic acid solution and then put on a Armstrong Clark semi-transparent stain. Now that the black marks have begun to show up, should I change this plan. Presumably by April/May (in BC) the marks will be much more widespread and i am concerned that they may not be removable by then. Any suggestions would be most welcome.
If you stain now then you will probably need to redo in the Spring. Best to wait. You can remove mold.
Western Red Cedar reacts with iron and steel when water is added to make black/grey stains. Did you use zinc screws and galvi. nails? Did you cut any steel or do any grinding where the black parts are? A new cedar deck should start to mold for a year at least. ( I live in BC)
We replaced all of our wooden deck boards and railing boards with wooden boards on a 15' x 30' deck. We waited 9 months and in the spring aggressively powerwashed the floor boards 🙁 ,and lightly powerwashed the rails. We then washed with soap and water and rinsed with a hose. It rained before staining. We waited two days for it to dry then started rolling the railing with CWF which took 3 days. Several days later I started rolling the floor boards which also took several days to complete. I believe there was sunshine with no humidity during this entire time period. The wood looked unevenly stained so I sealed it again. I didn't do this after the recommended 20 minutes, instead I resealed over a period of a few days beginning about 24 hours after initial application. The weather was the same. The deck still looked unevenly stained so I then spot stained those areas. I know the deck was completely sealed because water beaded up to the point where it was necessary sometimes to squeegie the deck to remove the water.
We have a fairly new cedar deck and we used a bleach based cleaner which ended up turning it white and tissue paper like. We ended up renting a power washer to clean it off which only did it partially..and we had to end up sanding the entire deck down by hand! We now have black spots on parts of the wood…what do we do about it now? Re clean it with Restore A deck or wet and forget etc… Please help!,
Depends on what the black spots are. If mold you could try the wet and forget. If rust from sanding, you need a wood brightener that is part of the RAD kit.
What do you mean rust from sanding? It looks like mold.. We will try the RAD and wet and forget. Thankyou!
Rust turns black in wood. Looks like small black spots and can look like mold.
Does the aluminum oxide sandpaper get embedded in the wood and rust? Or where does the rust come from? I can't find any info on the internet about it..
Rust can come from anything metal. Feel free to post a picture in our forum and we will let you know if rust or mold.
First time on site. Looks very informative. A very subjective question. I have 7 year old cedar deck that we are replacing with composite or PVC. Don't know which way to go. Have been looking at Gorilla Deck, Fiberon, Ever Grain, or Lumber Rock. Any advice would be greatly appreciated. I don't want WPC (Trex) as it tends to get swelling on the ends and can mold, I think. Also, we are going to use the existing joists but they have black mold growing on them. they were once stained with a water-based semi-transparent stain but now has mold on the surface. I was just going to mix bleach and water and spray it and leave it. What do you think?
Go with Azek or Timber Tech. Yes that will help with the mold.
I just bought a house that has a 10 year old deck that doesn't see a lot of sun and is in a damp area. It doesn't look like it's ever been cleaned or stained. It has mold/mildew engrained in the wood. I've cleaned it with bleach and TSP five times, power washed twice, and sanded once. Nothing works. The last time I cleaned it, the run off ran clear – no more dirt and mold is being removed. Not sure what to do to get the mold out of the wood. There are green patches everywhere. Is it even possible? If it's not possible, can I go ahead and seal it anyway or will that cause even more problems? What kind of sealant do you recommend for this?
If bleach did not get our the green algae and the black mold, nothing probably will. You might just need to go ahead and finish the process of staining.
I just spent the last two weeks stripping two year old Flood deck sealer off my redwood deck in Northern California. I then cleaned and brightened it. It was grueling work but now looks great. I bought Penofin Blue Label to put on it but am afraid of having to strip it again in two years. Penofin says I can just wash it in two or three years and reapply the stain. Is this true? I was so disappointed with the Flood product. it deteriorated quickly and had mold growing in many areas of my deck.
Not if it turns a dark brown or black color as we have seen Penofin do numerous times. You would need to strip it.
Hi! We recently moved into a home with a 10+ year deck. The wood seems to be in great condition (no cracks, holes etc), except it was covered in black mold. We used a power washer to remove the mold and now we are left with wood frays that are likely to cause splinters. Are there any products we could use to seal and stain the wood without having to sand it first?
Samantha, you cannot seal in splinters. I would lightly buff the deck with a floor buffer and sanding pad to smooth the surface than stain the deck.
I have a 3 year old cedar deck in Iowa. I used a clear oil based wood sealer 2 years ago after a light power wash and 3 day wait, nothing else. Since, it has become very dull and has the black mold or mildew on quite a bit of the deck surface and that smooth whiteish substance is visible when the deck is wet. I'm inclined to powerwash all this off, let it dry, then stain. I have read lots of conflicting items on whether power washing the mold off is good or bad…? After cleaning what is the best type of stain to use? Tint or clear. I like the wet look but did not get this effect with the last stuff I used.
Clint, we would use a stain stripper first and pressure wash off. Brighten when done with the stripper. This will remove the mold as well. Best to have a tint so you can prolong the Uv graying. Honey colors typically give the wet look you want.
Thanks.. I plan on using an oil base…right?
Correct!
I have a 14 year old cedar siding house I just bought that I would like to restain. I am planning on using TWP 100 for the stain, but I need some advice as to how to prep my siding. there is old stain on there, but I am not sure of the product that was used or how long ago it was stained. There is black mold and mildew present on the upper level and the boards close to the ground are very dull and dirty. I am not sure whether i should just use a cleaner and brightener or a stain remover and brightener?
For this it would be better to use a stain stripper, pressure wash off, than brighten.
We stained our 5 year old deck for the first time last fall. It was thoroughly cleaned per instructions and allowed to dry completely prior to applying the stain. A few months ago I noticed what appeared to be black mold showing up in several areas on the deck. Those areas are beginning to rot! How can this happen so quickly and what can I do to salvage the remaining boards? It seems to be spreading.
Sandra, can you tell if the mold is on top of the stain or growing \”in\” the stain? Nothing can stop mold from growing on top of a surface. This usually can be cleaned off easily. If the mold in growing in the stain then most likely you sued a oil based stain that is \”feeding\” the growth the of the mold.
what should someone do/use…exactly, in this situation ?
I have the SAME problem and we did use an oil base as highly suggested by a reputable paint store .
Strip off and start over if the mold is imbedded in the stain. If on top try a light cleaning to remove.
lovely…not a job I enjoy to start off with and this shouldn't have happened, IMHO.
Thanks for your help though, much appreciate it .
Should we go with a water base stain this next time after cleaning it?
Plan to clean it with just Clorox/water mix since the budget is very very tight here, is that allright ?
There are other smaller identical black spots popping up all over, looks like soot and the whole deck really needs to be started over and it's a big deck. :<(
Bleach will make the spots go away if mold. Use about 1 part beach to 5 parts water. It may bleach or lighten the stain though. The other reason for black spots are rust. Did you use anything metal on the wood for prep?
Have a deck similar to Big Al. Seven years old on the Southside of the house, lots of trees, stains, never sealed or cleaned. Plan to sell in two years. Immediate plan is to replace split boards, tighten it up, clean with some type of "stripper" and then power wash. Do the same thing again in two years, trying to replace any bad boards six months prior to the cleaning / power washing and then staining. Any problem with this plan? Somewhat concerned about the letting it set for two years without staining before repeating the process. Thanks.
Giggi, you should prep with a deck cleaner and brightener like the Restore A Deck Kits. No need for a stripper since it does not have an old coating. In addition it would be a good idea to have a coating applied now and again in 2 years.
Our wood deck is on the north side of house, no roof, gets lots of rain. At last cleaning, 4+- years ago, did not restain just applied sealer. So color and protection are uneven. We are not prepared to start over by stripping, sanding, etc.
I want to know what kind of cleaner will get rid of the dirt, mold and algae that have accumulated without compromising what protection still remains on the deck. Prefer something I can spray on and scrub with a push broom. (I have already used clorox and water, which helped with algae but not with dirt and mold.)
Susan C, since it is has been four years it is not possible to not compromise the old coating with a cleaner. Best to prep with a cleaner and brightener like the Restore A Deck System but you will need to apply a new sealer or stain after.
Wow!! What a great site – I have gleaned more information and learned more in this one site than the rest of the web combined regarding the care of my deck. Thanks to the site developers as well as the people who have contributed questions, answers and comments. This is a DIY'ers dream come true.
Thank you, thank you and finally thank you!!
Steve, we very much appreciate the praise!!
I have a pressure treated pine deck about 10 years old, never cleaned or sealed. It was a bit moldy and very gray so I applied a stripper (diluted sodium hydroxide with some surfracants) and then power washed. The power washer was a 4000 psi 3.5 gpm using the 40 degree (white) tip. I was a bit worried about getting too close to the wood so I washed from a foot or so away. It seemed to come clean but after drying the results were a bit uneven and there was quite a bit of very fine furring which appeared to maybe be the the remaining gray layer. So I power washed again this time closer to the wood. This time the wood looked very clean and new when finished AND the furring seems to be gone. My question is about the furring. My first assumption was that I had cleaned to close or with too much chemical but now I think maybe I just didn't get close enough the first time to actually remove the deteriorated top layer of wood, is this correct?
Big al, that is correct.
Thank you! WONDERFUL site you have here. I'll be back.
Sarah, is the mold on top of the stain or under the stain? Test by seeing it it wipes off with a wet rag.
Have applied Penofin red label stain onto new cedar deck 1 month later black mold on all surfaces what should
II do now?
We built our house one year ago. We had our second-story deck pressure washed and stained with Deckscapes at the end of May. The weather here has been unusually rainy for the past four months. The underside of the deck was not stained (we were told it was not recommended).
The top looks fine. However, the underside now looks terrible (there are large black spots and streaks (where the rain runs between boards) and smaller yellow spots, which I assume is mold and mildew. The person who stained it said he didn't know what to say except that it's been unusually rainy and that it would not hurt anything. However, it does not seem like it should look that bad, and the edges of some of the boards even look "jagged", I'm concerned they will rot.
We would like to know what we should do next. We are considering having the underside stained, but is there something specific we should use? Do we need to treat the bottom with something first?
Courtney, not much you can do except clean and brighten the wood for the prep than stain. It will still get black marks though on top of the stain from the rundown.
I just stained my mahogany deck with armstrong Clark mahogany after stripping the old failing stain, but it looks like black mold is already appearing in some areas. I was careful not to over apply, but after two days the stain is still not completely dry (but not shiny/tacky), and overall the stain penetration looks very uneven. After reading all the positive recommendations from pros about Armstrong Clark I am sure this is due to something I did wrong and not a product issue. Possibly it wasn't mixed well enough beforehand. I was thinking about lightly washing with a oxidized cleaner and then lightly restaining with a second coat, but not sure if this would make the problem worse by using too much stain too soon.
Adam, sounds like the wood took the stain unevenly. I would contact Armstrong for help with this.
Hi, we have a 2 year deck that is attached to our above ground pool. We sealed it with Thompsons water seal (honey gold not sure if that matters) https://www.thompsonswaterseal.com/waterproofing-p… .
Well, now we have what looks like black mold growing all over it, so before looking or reading anywhere I used some water + bleach and was able to get it off in some spots but most spots the water just beaded up. So once that appeared not to work I looked online and got even more confused on what to do. Could you please help me, also I'm really confused as to how the wood can grow anything with the chlorine from the pool that gets on it.
As my husband says I should get to the point lol
Ok so what to use to remove the water seal? Then what to remove the mold? Then what to use to prevent and protect the deck? And finally what kind of seal / stain would be better for chlorine traffic?
And we live in Georgia where humidity is almost a constant in the summer and the deck is in direct sunlight the majority of the day.
Thank you Jo-Ann
Jo-Ann, you will need to strip and possibly sand of the Thompsons deck stain. This should remove the mold as well but if not a good cleaning with a deck cleaner like Restore A Deck should help. Brighten the wood when done. Try Defy Extreme for this deck. Nothing will repel the chlorine but it will help prevent the mold.
live in sw Pa, deck is 3 yrs old, low to the ground, 6 inches one side to 18 inches opposite. used TWP stain. had to power wash black mold off 3 times last year, this year, 2 times and restained. it is back. will Behr premium help or hurt? or should we just let it go and start over with pavers/concrete?
Vicki, nothing can stop mold from growing on top of any surface and we strongly suggest avoiding Behr products.
We had our deck stained/sealed last year. Right after, our house was power-washed and the guy also lightly power washed the newly stained/sealed deck. We didn't know, but probably should have re-sealed it? About 3 weeks later, noticed black mold growing on the deck. Trying to take care of it this year. My question is…I don't want to remove the stain as it's still in very good condition…so should I just use a cleaner (not a brightener) and then a sealer on top of it?
Jen, sounds correct. Maybe some watered down bleach as well.