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.