Black spots
Brian Nystrom wrote:
Is this wood oak, by any chance? If so, the discoloration may not be due
to fungus/mold/mildew, but rather due to a chemical reaction between the
tannic acid in the wood and the metal in the screws. When done
intentionally, it's a technique known as "ebonizing".
If that's the case, bleach will work. Sam
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