What is the rule of thumb for painting over water stains? I have a two-story house, and my second-floor shower stall developed a leak, as evidenced by water dripping from the first floor ceiling. The leak is small–a few drops of water after every use–and I believe I have now fixed the source of the leak (failing, cracked grout).
Now that I’m preparing to repair the ceiling, I see that the water dripped out around a nail head which developed a quarter-sized black stain that I assume is black mold. A few inches away, the paint has cracked and is peeling along a two-foot-long drywall seam. I was hoping to simply remove the loose paint, clean the black area with some bleach, re-spackle and re-paint the existing drywall. Will this work or do you think that I should remove and replace the affected drywall to eliminate the black mold in the ceiling?
Before we get to painting over water stains, let's talk about mold. Mold needs three things to fester: air, food and water. While there is plenty of air and drywall is a terrific food for black mold, the fact that you have fixed the leak means that there should be no more moisture to feed a mold problem.
Secondly, the black mold that you think you have, may not in fact be black mold at all. While mold is possible, water stains--caused by the reaction of water with paper, paint and rust--can also form a black spot. Regardless, a quarter-sized area is nothing to worry about, even it is black mold, as long as the leak has been repaired.
As for painting over the water stain, my suggestion is to most certainly remove the loose paint as you suggested. Then wash the area down with a bleach-and-water solution just to make sure any mold spores have been neutralized. The next step is the most important: paint the stained area with an oil-based primer like KILZ. This will seal the stained surface and prevent it from leaching through to the top layer after you paint. For the best results, prime the entire area and not just the spots that have been impacted by the leak and resulting water stains.