On Friday, January 22, 2016 at 2:35:45 PM UTC-5, wrote:
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

we had and fixed that problem here, and its been fine for about 20 years.

remove everything loose, really work at it. then wash with tsp and rinse well. let dry for a couple days. then paint with a bonding agent so the next layer sticks well..

then skim coat with drywall mud. let dry well.

then paint.......give it 2 coats

like i said near 20 years ago and zero problems.

avoid sanding because of lead based paint, and possible asbestos