Wood finishes are water resistant -- which is one of the main reasons for using them -- but few are waterproof. Standing water, steam and even persistently high humidity can turn many finishes cloudy -- a type of discoloration caused by moisture seeping into the finish. When water seeps all the way through the finish and penetrates the wood, a different kind of stain results. Some wood species, such as fir, may simply turn dark, but other species with a high tannin content -- primarily oak -- can turn black, as if they were moldy. Wet oak often is moldy, and the mold contributes to the black discoloration caused by tannin reactions with minerals in the water.