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How to remove black spots from your composite decking

This stunning lake boardwalk covered with composite decking gets covered with black spots. They can be prevented. (Tim Carter)

I’ve got 1,500 square feet of lakefront boardwalk made from composite decking. It started developing black spots within two years of the installation. I’m mad as heck about this, as I thought I’d have a maintenance-free deck for life. I’m not able to get any help from the manufacturer or from the big box retailer that I bought it from. It’s going to cost me $20,000 to replace it. Can you help me at all? — Jim C., Eaton, Ohio

I’ve got great news for you. You’re not going to have to replace that composite decking. I’ve also got a small dose of tough love, too.

Let’s start with a story. I remember years ago when two products were introduced, both had to do with decks. I can clearly remember seeing a counter placard at a lumber company in Cincinnati that said, “Rot-Proof and Maintenance-FREE Deck Lumber.” It was advertising the new copper-chromate arsenic (CCA) lumber for deck framing and deck boards.

We all know how that turned out. You did have to seal the decks every two years and the lumber did rot.

I also remember similar early claims by the manufacturers of composite decking material. As a member of the working press, I was inundated for years by the public relations companies spreading the word about these miracle products.

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My own home in New Hampshire, which I did not build, had this first-generation composite decking that did fade and did develop deep black spots. It also allowed algae to grow on it. Millions of other homeowners did the slow burn like you’re doing as they discovered that you do have to maintain the decking.

There’s no such thing as maintenance-free when it comes to any outdoor product. The reasons are many. For starters, the sun’s ultraviolet (UV) rays are so powerful they break apart metal atomic bonds. So simple paint, plastic or wood molecules and fibers are child’s play for the sun.

Mother Nature is also quite adept at showering everything outdoors with mold spores, algae, dirt, diesel soot, dust, etc. Add water to this slew of ingredients and you’ve got perfect conditions for mildew, mold and algae growth on any surface.

At the very least, maintenance involves periodic cleaning. You can clean some things with regular liquid dish soap and water. Tougher stains from barbecue grills, tree sap, mold, mildew and algae may need more powerful powdered oxygen bleach you mix with water.

The issue in your case is that your composite decking, and several other brands, contain untreated lumber fibers that are encapsulated in recycled plastic. However, not all the wood gets coated with the plastic and, as I mentioned earlier, the sun breaks apart the plastic, exposing the wood.

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Mix wood, spores and water and you get black spots. You can clean them off, but the spots come back because the mold and mildew spores are constantly floating down onto the deck and you have a nearly constant supply of water. Overnight dew, which covers everything outdoors many days of the year, is perhaps the biggest source of the problem.

I think you can get some peace of mind, but it’s going to take a little periodic work on your part. Fortunately it’s easy to do.

Years ago I became fascinated with clipper ships. I have two boxes filled with small parts of two ships I intend to build when I’m retired — the Cutty Sark and the Thermopylae. Both of these tea clippers had copper plating on their hulls.

Copper is a natural biocide. It prevented barnacles and other organisms from growing on the wood hulls of these fast ships. These nasty growths would slow other ships and reduce the owners’ profits.

You can introduce this copper to your boardwalk in a way that should not harm anything that’s next to the boardwalk. I’d clean the decking first and then apply a mist of copper sulfate solution to the decking.

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Copper sulfate is readily available, and farmers have used it for decades. It controls fungus diseases, makes up for copper deficiencies in animals and other things. If you drink wine, then you have to appreciate copper sulfate! Vintners use it.

I’d mix up a solution and put it in a garden hand-pump sprayer. Set the nozzle tip to a fine mist. On a sunny warm day, spray the surface of the decking. Minimize overspray. The solution will soak into the wood fibers and possibly the tiny voids of the decking.

The only unknown is how often you need to respray. It could be once every six weeks, but it’s far easier to do than scrubbing that decking. Good luck!

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