I have a ~100 year old house. In a small spare bedroom, the ceilings were getting a little wavy with age, and several of the walls had cracks/damage, so I decided to skimcoat. I'd noticed that in a few spots the paint on the ceiling(but not the walls) appeared to be peeling and revealing some sort of black coating underneath. I figured it was probably from painting the ceiling black at some point(for who knows what reason), but I wasn't too concerned. It is NOT mold, as it appears to be a uniform coating of some sort. I scraped/sanded any areas that appeared loose, to the point where it all appeared stable.
I've skimcoating the entire room at this point, thickly on the walls but fairly thin across most of the ceiling. Tonight I tried rolling primer on the ceiling. As I did, the existing paint/plaster began peeling off in SHEETs, revealing the black coating underneath. It's coming up in areas that previous had not been peeling, so I know it wasn't loose edges there.
On closer inspection, the black coating seems fairly hard, almost with a rough texture, not unlike chalkboard but with a pebbled finish. It seems a little glossy as well. If I try to scrape it, it almost seems like it has the outer black coating, a thin harder coating, and then plaster underneath.
Any ideas? I'm guessing my best recourse at this point is probably just drywall over the entire thing, but I'm open to ideas...