We want to repaint a plastered room. It has 2 (3?) previous coats of
uncertain age. Some paint is coming off, at least partly due to water
from a long-ago roof repair. I scraped it pretty aggressively. In
some areas, I got down to the plaster; in others, the paint seems
intact.

Where I did scrape paint off, I find that I can attack the exposed
edge with a razor blade, and pop the paint off in small patches.
Mostly, this is just the top coat, though sometimes, it gets down to
bare plaster. It's not clear how far I could go with this - at the
least, I can get off a LOT more than the scraper did.

The question is, whether this is 'normal'? It doesn't seem like
well-bonded paint, but maybe it's OK? It would be a PITA to do the
razor thing on the whole room, but there's no point in new paint
that's going to come off, either.

Thanks