I'm stripping my 80 year old plaster walls to repaint. There's a top layer of paint, then two layers of wallpaper. In 90% of the rooms the paint and wallpaper comes right off. In some patches though, the paint won't budge. I've tried scraping it, removing it with wallpaper remover concentrate, and even using paint thinner but it appears it's painted straight onto the plaster almost. Can I just sand these areas down some so they're flat and then prime right over them? Or do I need to completely remove all old paint? Because in one room it's fairly prominent and these old paint areas cover about 20% of the walls.

Thanks for any help!

 

No need to remove all the paint. Just sand them flush, or skim coat the edges to make them appear flush. Make sure to prime the patched areas, as these will absorb the paint at a different rate than the rest of the walls.