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How To Clean and Seal Composite Decking

Written by  Dave Barnes

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 probably paid a little more for your lumber and expected it to be maintenance free and to 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.

composite-deck-mold

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 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. Without some type of mildewcide to prevent its growth, mildew flourishes during times of high 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 clean the composite deck boards 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 it will also do it much faster. Just remember, you can't remove anything left on the deck once the project is finished. So take your time to thoroughly clean the deck.

Use a Safe Cleaner. Avoid using Bleach.

The cleaning process will work best if you initially select the right cleaner. Some folks use common laundry bleach. I hate bleach because it kills plants and vegetation, and it's terrible to work with. While it kills the mildew spores, it 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 it is highly effective at not only killing the mildew spores, but also at removing them from the surface of the decking so they don't become a food source for additional mildew growth. A short video on how to properly use the cleaner can be viewed here.

Seal the Deck

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

Choosing the Right Composite Decking Stain is the Key

The sealer that you select will determine how satisfied you are with your project once it's complete. Here are a few things you should look for when selecting your product:

First, you want a water-based sealer as opposed to a solvent. These are easier to work with and have two main advantages over solvent-based materials. Solvent-based products, or alkyds, will typically amber the color of the surface; many of the water-based products are crystal clear and will not alter the color of the wood. The alkyd resins in solvent-based products will also 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, which 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 like me who 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 before you start and read it. The directions will make you aware of any particulars of the product that can affect your outcome, such as: What's the dry time? How many coats should you apply? How much time between coats? Is there a critical recoat time? Any number of things can be on 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 roll 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 using a car-wash style brush on a pole. This is my favorite method as it does an exceptional job and 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 stain interior wood, do the same if you over-apply sealer on exterior composite lumber. The reasons are 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. Secondly, 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 while the sealer has a chance to cure out. Come back tomorrow to 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 returning, 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 the mildew problem when you chose to build your deck out of composite lumber, but with a small amount of maintenance, you can still end up with a terrific looking deck to entertain on and enjoy. Please share any comments about your project in the comments section below.