This post was updated on February 27, 2024
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.
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
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.
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.