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Redwood deck unleashes black soot attack

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Q: My redwood deck, which has never had any type of waterproofing or other protective coating put on it, always turns black after I power wash it. Whatever this black stuff is, it comes off on shoes, socks, skin, material–I can't even lean on the railing for fear of getting black soot on my clothes. I have had several redwood decks in the past and have never had this happen before. What is it? What causes it? What can I do to prevent it? –John, San Francisco A: Without seeing the deck, we believe your redwood deck is leeching tannin or you have a mildew problem. Tannin, or tannic acid, is the substance in the redwood that makes it resistant to fungus and termites. It's black. If the deck is in a protected area that gets little sun, a fungus is the more likely culprit. Kevin had a similar problem in Alameda, Calif., with a deck that was on the north side of his house. It got some sun, but in winter the black slime came out. At one point he even started to grow a nice...