My contractor suggested I apply ¼” sheetrock to a mildewing bathroom wall. The mildew was caused by a bad reaction between oil and water-based paints, and it’s a very damp room. He claims just scraping off the old paint, priming, and painting will not stop the problem. Is this true, or is there another way to handle the issue?
Milos, I agree with your contractor that just scraping off the old paint, priming, and painting will not stop the problem, but I don't believe that covering it with new sheetrock will cure the issue either. I have never heard of a reaction between latex and oil-based paints causing mildew to grow, but that's not to say that it couldn't happen. However, considering the issue is in your bathroom and that you say the room is very damp, I suggest looking for another culprit that could be creating the problem.
Mildew and mold need two things to flourish: moisture and air. As there are several plumbing supply and drain lines in just about every bathroom, even a minor leak in any one of them can create the perfect environment for the fungus to grow. It might be a good idea to have a plumbing contractor check for leaks before doing any type of repair work. A minor leak inside a wall can remain hidden for quite a while, but may still be causing damage to your home even beyond the mildew.
You don't mention how often the bathroom gets used for shower and baths, but if it's frequently and the space doesn't have adequate ventilation, that can also be a cause of dampness problems. If you have a bathroom fan, it might not be working properly or the ductwork could be blocked. And if the room doesn't have a fan, you may want to think about having one installed.
Mildew is one of those problems that usually returns if the issue that's causing it isn't corrected. That's why I think simply covering over the damaged sheetrock with new board isn't the best course of action - the problem is still there, you just can't see it as well. And don't forget that mildew may pose a health hazard to your family.
I would correct the problem creating the dampness and then have a mold remediation company inspect the room. Any materials that have been exposed to the fungus should be removed or treated and then you can proceed with repairs. My guess is that the mold remediation company will recommend that the contaminated sheetrock be taken out before any new boards are installed.