I have drywall in my bathroom that has started to flake and has become damaged over a few decades of use over a shower that didn't have an extractor fan.
I'm currently in the process of planning out the new extractor fan project, but I'm not sure what to do about the drywall. Here is a picture of the situation:
I've seen videos of drywall failure modes similar to this, and their solution was to use a 6" putty knife to scrape off the hanging drywall until the remainder was totally flat and then coat it with 20-minute joint compound.
I'm not entirely certain if that would work here because (As others pointed out in similar questions) joint compound is water soluble.
What is the best solution to this problem, and what tools would I need to implement it? (I'm kind of new to drywall, although I do have a 6"/12" steel putty knifes and a mud pan)
NOTE: I think that this differs from (repair drywall in the shower) as my drywall is not water damaged in the same way, and thus likely won't have the same answer (To the best of my knowledge), and is also different from (How do I repair drywall around a bathtub?) due to not being crumbly and other types of damage to the drywall being done.
Best Answer
To my mind the solution isn't dependent on your choice of joint compound (though topping compounds lack the vinyl component which can help with bonding). Even if you use concrete to coat it, what's behind it is still susceptible to water damage. Instead, moisture management and a good primer/paint coating is important. Your walls simply shouldn't be exposed to prolonged moisture.
I'd scrape away all loose material and finish the surface with whatever mud suits your fancy. Prime it with something that seals well, and use a high-quality topcoat that claims to bond well and protect against moisture damage.
An exhaust fan should be on a modern digital timer. Run it for at least two hours after a shower, as the moisture stuck to the tub and your towels takes a while to evaporate.