 All right, Nicole reached out to Team Money Pit saying, what is the rule of thumb for painting over water stains? I recently fixed a leaking second floor shower, but I've noticed that there's a black stain around a nail hole where the leak occurred, which I assume is mold. It could be rust, though, too, right? It could be rust, or it could be another reaction of the water and the drywall and the paint. It often causes, and the nail, of course, often causes those sort of blackish stains. In fact, they don't think it is rust.