Keeping your deck clean and pristine will not only make it more appealing, cleaning the wood will help it last longer and protect it from harmful substances. Black spots on your deck might be fungi decaying the wood or tannin stains. Use the right chemical to clean the wood deck and remove these black spots.
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Oxalic acid, a common wood deck cleaner, is sometimes referred to as a wood brightener or whitener, as it can lighten the color of the wood. Don't use oxalic acid, therefore, if you want to preserve the darkness of your wood. It is effective in removing fungus or tannin stains. To use the acid, mix with water according to the manufacturer's directions. Apply it to the wood and let it sit for several minutes before scrubbing away the black spots. Rinse the deck when the stains are gone.
Detergent is effective in removing light black spots caused by tannin or mold. Mix several drops of uid dish detergent with water until the water is sudsy. Dip a stuff-bristled brush into the soapy water and scrub the black spots until they are gone. Rinse the wooden deck with clear water and let it dry completely.
Bleach is one of the best solutions for removing black mold and mildew spots on your wooden decking. Mold and mildew grow when the wood is wet and doesn't get a chance to dry often. Mix 1 cup of bleach into 1 gallon of water. Dip a scrub brush into the solution and spread it over the deck. Let it sit for several minutes before scrubbing up the mold. Rinse the deck when the black spots are gone.
Trisodium phosphate (TSP), another effective solution, is a powder mixed with water to form a cleaning solution that quickly cleans wood without damaging it. It is not as strong as oxalic acid and will not lighten the wood. To use it, mix 8 to 10 tbsp. of TSP with 1 cup of bleach and 1 gallon of water. Brush the solution into the stains and rinse the area with water.