Black Mold & Mildew and Deck Stains  4.4/5 (23)

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

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

Mildew on Decks

Have a question or comment about black mold and your deck? Please ask below.

Please Rate This. You may also post comments or ask questions below.

author avatar
Scott Paul ~ Restoring Wood & Decks Since 1993 Owner
As an article and comment contributor to the site, Scott has been around the pressure washing industry since attending college. In 1993 he started his first company called Oakland Pressure Wash specializing in exterior pressure washing and deck staining. That company evolved into OPW L.L.C. shortly thereafter concentrating more on exterior wood and deck restoration. Scott and his Deck Cleaning Michigan company have restored over 10,000 decks in the Metro Detroit area since the early years. He has become an authority in the deck restoration industry and has contributed to numerous wood restoration forums and informative sites. All the products he suggests through this site are sold through online sites and in retail stores, allowing the consumer to choose their own means of purchase. Scott’s eCommerce sites do sell many top brands he endorses and if you appreciate any of the help he has offered then feel free to purchase from one of them.

Related Deck Stain Help Articles & Reviews

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Peter
Peter
11 months ago

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.

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Al Wolfe
Al Wolfe
11 months ago

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?

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Jack
Jack
1 year ago

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.

Trent
Trent
1 year ago

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!

Travis Williams
Travis Williams
2 years ago

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.

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Travis Williams
Travis Williams
2 years ago

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?

isaac reich
isaac reich
2 years ago

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!

John
John
3 years ago

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?

John
John
3 years ago

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!

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John
John
3 years ago

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.

John
John
3 years ago

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.

John
John
3 years ago

. . .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??

donna
donna
3 years ago

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.

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Pamela
Pamela
3 years ago

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?

Kathy Alexander
Kathy Alexander
3 years ago

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.

Collette Kelly
Collette Kelly
3 years ago

I was wondering for the black mold on my unstained deck if I could use a solution of bleach and water!

Ashley
Ashley
4 years ago

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

Ashley
Ashley
4 years ago

Brighter after cleaner or sanding?
And is it ok to wait a week and then stain?

Susan Kleinstein
Susan Kleinstein
4 years ago

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!

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Kathy Alexander
Kathy Alexander
3 years ago

Nope, it doesn’t look like dirt! Mine looks the same and deck isn’t affected under our porch cover.

Susan
Susan
4 years ago

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.
~

Susan
Susan
4 years ago

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?

Susan
Susan
4 years ago

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.

Susan
Susan
4 years ago

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,

Susan
Susan
4 years ago

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?

Josh
Josh
4 years ago

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.

Evan
Evan
4 years ago

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

Gary H McCoy
Gary H McCoy
4 years ago

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

Gary H McCoy
Gary H McCoy
4 years ago

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.

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Tom Kneer
Tom Kneer
4 years ago

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????

Mick
Mick
4 years ago

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?

Mick
Mick
4 years ago

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?

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Mick
Mick
4 years ago

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?

Mick
Mick
4 years ago

I posted three above. Here’s another. Seems to mellow a bit after drying, but not much

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Mick
Mick
4 years ago
Reply to  Mick

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.

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Mick
Mick
4 years ago

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.

Aaron
Aaron
4 years ago

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

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billy bergh
billy bergh
4 years ago

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…

Cranky Frankie
Cranky Frankie
5 years ago

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

Jennifer
Jennifer
5 years ago

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!

Jennifer
Jennifer
5 years ago

Will this then help it stay looking good? How often should we do this? Meaning, yearly, every two years, etc. ?

Caroline Strong
Caroline Strong
5 years ago

We have a large black greasy/sticky area on our deck. It is near a hummingbird feeder. Could there be a connection?

Caroline Strong
Caroline Strong
5 years ago

Thank you for your quick response and advice.

Chuck
Chuck
5 years ago

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?

Chuck
Chuck
5 years ago

Thanks. Is there a surface spray mildewcide you recommend that could be applied periodically without degrading the TWP 100?

Linda
Linda
5 years ago

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

Linda
Linda
5 years ago

See picture below.

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