14 years ago moved to "new" 50 year old house with plaster board / plaster. after cleaning with tsp we painted everything with zinzer coverstain oil bond coat and than painted with devoe latex. in recent years have had some pealing of the paint two or three layers below what we painted (awful colors) in some patches. How far do I need to chip / scrape pack before filling and painting. I have all the loose stuff off but if I work at it a bit I can always get more off. It appears that the layer I am at now is the top plaster coat, very smooth.
It's hard to really explain, but keep scraping till the surface puts up a good fight. Plaster will dry out over time and loose adhesion with the first coat of paint, but typicly it should last a lot longer than 50 years.