How To Clean and Seal Composite Decking

Follow these simple steps to clean mold on composite decking (cleaning Trex®). When you made the decision to build your deck using composite lumber, you paid a little more for your lumber, expecting that it would be maintenance free and maintain its beautiful appearance for years to come. If your deck is constructed of older composite decking (installed prior to mid-2000), then you probably know what a big problem unsightly mildew growth has become.

Many of these older composite lumber products were manufactured without any type of mildewcide in their formulas and now the problems have become obvious: unsightly black mildew spots everywhere.

The reason why mildew grows on composite decking is really pretty simple. Composite lumber is made up of a combination of both plastics and wood fiber/flour. The wood flour (like saw dust) is a food source for the mildew to grow on and without some type of mildewcide to prevent its growth, it will flourish during times of higher temperatures and moisture. But the fix is pretty easy if you just follow a few steps.

Composite Deck Cleaning (Trex® Cleaning)

The first step is to remove any mildew, dirt, or stains that are currently on the deck. For cleaning, I highly recommend using a power washer on composite decks as opposed to just a garden hose and a scrub brush. A power washer will not only help get things cleaner than scrubbing by hand, but will also do it much faster. Just remember, anything that is not removed by cleaning will not be able to be removed once the project is finished. So take your time and thoroughly clean the deck.

Use a “Safe” Cleaner. Avoid using Bleach.

The cleaning process will go best if you select the right cleaner to begin with. Some folks use common laundry bleach. I hate bleach because it kills plants and vegetation and it’s terrible to work with. It will kill the mildew spores, but doesn’t necessarily remove them from the surface of the wood. Instead, I recommend using DEFY Composite Deck Cleaner. This cleaner is specifically formulated for cleaning composite decking, including Trex. It is an environmentally safe product that will not harm plant life or vegetation and is highly effective at killing and removing mildew spored from the surface of the decking so they don’t become a food source for additional mildew growth.

Seal the Deck

You don’t have to seal the surface once it’s been cleaned, but I suggest you do. By properly sealing or staining composite decking, you could get several years of use out of it before you need to clean it again. If you decide not to seal the deck, you will most likely need to plan on cleaning it again every year or so.

Choosing the Right Composite Decking Stain is the Key

The sealer that you select will determine your satisfaction with your project once it’s complete. There are a few things you should look for when selecting your product.

First, you want a water-based sealer as opposed to a solvent. In addition to simply being easier to work with, there are two main advantages in selecting a water-based product. Solvent-based products, or alkyds, will typically amber the color of the surface, whereas many of the water-based products are crystal clear and will not alter the color of the wood. Additionally, the alkyd resins in solvent-based products will serve as a food source for mold, mildew, and algae (what you’re trying to eliminate). Some water-based products (not all) utilize synthetic resins; these offer no food value for future mildew growth. Lastly, selecting a product with zinc nano-particles is especially beneficial. Zinc is a natural combatant to mold growth and will have a profound effect on delaying the re-growth of mildew. Consider using DEFY Composite Deck Sealer – Crystal Clear; it has all of the above characteristics.

Follow Directions

All of you out there who are like me and don’t need to read the directions…READ THE DIRECTIONS. Everything that you need to know about the product is on the back of the can. Take a few minutes to read it before you start. The directions will make you aware of any particulars to the product that can affect how the job turns out, such as: What’s the dry time? How many coats should you apply? How much time between coats? Is there a critical recoat time? There can be any number of things that you can learn from reading the back of the can. Take a minute and read the directions.

Brushing is Critical

Regardless of how you apply the sealer—rolling or spraying—it absolutely, 100% must be brushed. If you rolled it on first, simply back brush it in after it’s rolled on, but while it is still wet. The same goes for spraying. But why go to all of that trouble?

It’s a simple explanation. When you brush the sealer, the brush creates friction on the surface, breaks the surface tension of the wood, and works the sealer into the pores of the wood. The more sealer that you can get the wood to absorb, the longer your project will last. Check out this video that shows how to apply deck stains quickly by using a car-wash style brush on a pole. This is my favorite method as it does an exceptional job and really makes the job go fast.

Wipe Off Any Excess

This one might throw you a little, but it’s important. If you apply too much sealer to the wood (more sealer than what the wood can absorb), wipe off the excess. We don’t think twice about wiping off excess stain when we are staining interior wood. My advice is to do the same if you over-apply sealer on exterior composite lumber. The reason is simple. First is aesthetics. When you apply too much deck sealer and it puddles on the surface and dries, it looks bad. It will leave shiny spots on the surface. Second, over time those spots are likely to flake and peel. Only apply as much deck sealer as the composite lumber can easily absorb.

Be Patient and Let it Cure

At this point the deck is done; if you’ve taken your time and followed these steps, you’ve got a great looking project. Don’t mess up now. Let things dry. Leave the furniture in the yard and the dog in the house so the sealer has a chance to cure out. Come back tomorrow and put things back together. You’ll be glad that you waited a day to do it.

Now Just Maintain It

We all know that nothing lasts forever and this will be true with your deck as well. But if you catch things early, maintaining your deck’s appearance is pretty easy. At the first sign of any mildew coming back, simply wash the surface of the deck with DEFY Composite Deck Cleaner and a scrub brush, hose it off, and let it dry. Then apply a single maintenance coat of clear sealer. It’s just that easy.

You may not have bargained for mildew when you chose to build your deck with composite lumber, but with a small amount of maintenance you can still end up with a terrific looking deck to entertain on and enjoy.

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  1. melissa says:

    I purchased Sherwin williams super deck solid deck stain love the product but even though it has a sealer in it don’t like how it scuffs. Can I use a Thomson water sealer over it?

    • defyadmin says:

      Melissa, you’ll have to check with Thompson’s, but most likely not. Most stain manufacturers recommend removing previous coatings as they will prevent the new coating from penetrating into the wood pores. If you were going to use our DEFY Extreme Clear, we would recommend removing the Sherwin Williams product first. With that being said, solid color stains are difficult to remove. Unless you want to sand the deck down to bare wood, or use something like a paint stripper, you may want to stick with a solid color stain.

  2. I have just cleaned my Trex deck ( approx. 12 yrs. old) with a bleach, water, small amount of Tide detergent spray, followed by a LOT of water to rinse it off. It looks clean and now I want to seal it. Your Composite Deck Sealer sounds like a winner! Would I have to clean the deck again before applying the sealer?

    • defyadmin says:

      Priscilla, you should be ok to seal it as long as the surface is completely clean and free of any mold, mildew, or previous coatings.

  3. Sherri says:

    I have purchased composite decking that does not have any coating. I’m wondering if I should apply the DEFY Composite Deck Sealer onto the boards, both sides, before installing.

    Would you recommend this method?

    • defyadmin says:

      Sherri, you can apply DEFY Composite Deck Sealer before installing or after, it’s up to you. Typically it’s not necessary to apply to the underneath sides of the boards, but it won’t hurt anything either.

  4. David W says:

    I have a 9 year old Trex Select composite deck, original saddle color, but faded to a very light milky tan, with lots of black mildew/algae discoloration, even after my initial attempt at a vigorous brush cleaning using an alkaline detergent from Wal-Mart, Krud Kutter. Will the Defy deck cleaner likely get the black stains out of the composite deck pores that still remain? Will a power washer also be needed? If the stains can be removed and I then apply the clear Defy sealer, what likely will be my deck color? When dry the current light tan color becomes much closer to the original saddle when wet with water, so would my Defy sealed deck once dried be colored more like it is now when dry or wet? Another option I’d like to pursue is changing colors from the tan/cedar tones to a weathered gray. Does Defy offer a product to allow me to stain my composite deck, such as Defy Extreme or another product, and therefore change colors? If so, what do you suggest? Last question, I am replacing an older portion of wooden deck with Trex. It comes in 3 grades (Select, Enhance and Transcends). Should I seal the new deck boards with clear Defy Sealer before installed and avoid the past issues of black mildew/algae build up or is it necessary with newer Trex composites? I really like what I read about your products. Thanks for any information you can provide.

    • defyadmin says:

      David, yes our Composite Deck Cleaner will work, but you’ll need to use a power washer as this will help remove all of the staining. Our Composite Deck Sealer goes on milky white and dries clear. You can order a small 1 oz. sample of the clear to see exactly what to expect at You could use DEFY Extreme as well, but you may want to order the Driftwood Gray sample color as well to make sure you like the way it looks over top of your existing saddle color. Concerning the newer composites, it may not be necessary to seal them as it was the older Trex decks that had all the mold problems. It wouldn’t hurt to apply our sealer to the newer boards, especially if you’re using the gray color, so that all the boards match.

      • David says:

        Thanks for your response. I ordered the Defy composite deck cleaner (2.25 lb tub) and tried it today as directed on my Trex composite deck after already have used the Krud Kutter to clean the surface. Two applications setting for over 20 minutes each and scrubbed vigorously with a brush. The black algae is apparently growing down in the deck board pores, not on top, because nothing changed. It still looks bad. I’m not sure a pressure washer is the sole difference and as I don’t own one, I don’t want to buy one just to find out. Going to keep shopping for products that might be more robust that the Defy cleaner. Also I’m afraid if I do not kill this black algae first that no matter what I might seal or stain the composite with, that the black algae will come through. At least that is what I read. Is that your experience too?
        Again, thanks for the assistance.

        • defyadmin says:

          David, sorry to hear that you weren’t able to remove it. My guess is that a power washer would have helped but I certainly understand not wanting to go and purchase one as they’re not cheap. As an alternative to purchasing a power washer, you may consider renting one as there are paint/hardware stores that usually have them available. You are correct though, you shouldn’t stain or seal over top of the black algae as it will cause problems for the sealer.

  5. Glenn says:

    Hi I am looking for a sealer to put over my dark grey composite plastic decking were the builders have scuffed it what do you recommend

    • defyadmin says:

      Glenn, if the composite deck is newer, you don’t want to seal it as the sealer won’t be able to penetrate. If it’s an older composite deck that’s absorbing water, you can use our DEFY Extreme Composite Deck Sealer. To make sure it’s able to be sealed just sprinkle a few drops of water on the deck. If it soaks in, it can be sealed. If the water just beads on the surface, then you shouldn’t apply a sealer as it won’t soak in. Keep in mind it’s a clear sealer so it may not hide the scuff marks.

  6. Fluffy says:

    Hi my composite decking is about 10 years old. I scrubbed all the green mold off last summer and deck was beautiful. This spring has been very wet. I did your water test of a few drops and it does head up. Should I just clean or clean and use the defy sealer?

    • defyadmin says:

      In doing your water test, make sure you just sprinkle a few droplets of water and watch to see how long it takes them to absorb. If it’s still beading water, then you’ll just want to clean the deck. If you apply the sealer while the surface is still beading water, then the sealer may end up peeling.

  7. Devy Fleishman says:

    I have a six year old chestnut color premium tree deck. It seems to be fading and looks dull. We power was at least once a year, but it still looks dull and faded in certain areas. Is there any sealer that will bring some life back into it?
    Thank you,

    • defyadmin says:

      Is the composite wood absorbing water? Sprinkle a few droplets of water and watch to see how long it takes them to absorb. If water is able to soak in after a few seconds, then apply DEFY Composite Deck Sealer. This will seal it as well as bring a little life back into it. If the wood repels water, then don’t apply a sealer as it may peel.

  8. David says:

    After sealing a large composite deck is it okay to walk on it to wipe off the excess?

    • defyadmin says:

      David, yes you can, but ideally try and wipe off the excess as you go. This will help prevent any footprints in the finish.

  9. Arnold Casey in Elizabethtown KY says:

    I have a composite 2004 deck that needs help. The color is light gray with dark mold spots in the walking area. I clean the deck every spring the spots disappear for a while, here it is July and the dark spots have appeared again.
    I have contacted the Local stores that sell engineered wood decking and they say you should not seal engineered wood it’s not recommended. To me sealing is the only solution to eliminate the mold from growing and turning dark.
    I have two power sprayers, 1850 PSI and a gas 3000 PSI power sprayer I could use either on although the electric is easier to handle for a 78 year old man. I have read that you should not use a power sprayer on engineered wood as it would take off the protective coating.
    I don’t have to worry about protective coating because there isn’t any. I put water in various areas an there was no area that bubbled only absorbed the water. In the past I have used a garden sprayer and worked in areas about 10 ft. square and rinsed of because instruction on container was not to let dry….. What would you recommend???

    • defyadmin says:

      Arnold, around the time your composite deck was made, some manufacturers (Trex) starting cutting their costs and using cheaper materials which later turned into a class action lawsuit because of all the defects and problems (specifically with mold). It sounds like may have one of those decks. We actually created 2 products to help solve your problem. First use DEFY Composite Deck Cleaner to clean your deck. You can apply it in sections or over the whole deck. If it starts to dry out, just mist it with a hose, or apply more cleaner. If you do this on a cloudy day, it will help it to not dry out too quickly. Rinse it with a power washer (electric should be fine), and then allow it do dry out. On newer composite decks, you shouldn’t power wash them as there is a coating, but in your case, that coating is probably not there. The next step is to seal the surface with DEFY Extreme Composite Deck Sealer. This should drastically reduce any mold issues as the sealer will prevent water from penetrating into the wood fibers. You can easily apply the sealer with a pump-up sprayer. Depending on how much foot traffic your deck gets, you may want to apply a maintenance coat of sealer every 1-2 years. Hope this helps, good luck!

  10. chris says:

    I have a deck that was installed about 15 years ago and has held up quite well. Its the Good life Villa decking. Id like to slightly change the color and was wondering if there’s a product that can achieve that. Im not sure if there’s a water or oil base product that will work


    • defyadmin says:

      Chris, we do have a Composite Deck Sealer that’s designed for weathered composite lumber. It is a clear sealer though, so it’s not going to change the color of the deck. One of the disadvantages of composite lumber is once you choose a color, you can’t change it.

  11. Nick says:

    Oh dear. I read a blog before to use Bleach on cleaning the deck. Your blog says I should stop doing it. I had a problem with bleach. It leaves stains after applying it. Thank you for the tips.

    • defyadmin says:

      Yeah it’s not good, plus it will kill any plants or vegetation surrounding the deck. An oxygenated bleach is a safer alternative and it works without causing any damage.

  12. Vicki says:

    Hi our composite deck not only has mildew in areas but has also lost its smooth texture and color. It seems to be waterlogged-doesn’t repel at all. Is there a product that can help restore it? We don’t know the exact year (around 2005) it was installed or the manufacturer.

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