I removed several coats of wallpaper from plaster walls, and then washed off glue residue with trisodium phosphate (TSP) solution. TSP is widely available at hardware stores. I had a lot of wall space to cover, so applied TSP solution with a garden sprayer, then used big sponge/clear water to clean. After walls were very dry, I primed with latex primer (although I've heard that alkyd primer may have been better on plaster). Just finished painting today and seems fine.