If there is a finish on the window sill, then you must remove it. If the offending area is not too large, you could sand the finish off. Then treat the area with liquid bleach. Oak is particularly stubborn and might require more than one treatment. Wipe dry with alcohol. This process will bleach out any stain that was added to the wood so, you might have to wipe on a stain to match the surrounding color. Add a polyurethane finish to prevent this in the future. --Previous Message-- : I have a dark water stain on my oak bay window : ledge. How do I go about removing it. :