I added a large deck to my home a few years ago, and shortly thereafter, black, oily stains developped on several of the boards. They are quite unsightly, and I would like to get rid of them. I have been told they may be a form of mildew, and that a solution of bleach, water and soap will solve my problem. However, I have also been told that bleach will harm the wood. I am also concerned that the bleach may cause discoloration.