I'm planning to paint our powder room. However, there are some challenges to smoothing the walls. The room used to have a window, which was removed in the course of renovations last year. The contractor plastered the area well, except that he plastered over the wallpaper around the window. I've removed the wallpaper, but when I sand the plaster it crumbles like the face of a cliff, rather than becoming a smooth low grade hill. What's the best way to deal with this?

Second, the wallpaper paste is remarkably well attched to the wall. Several rounds of DIF have failed to budge most of it, leaving some bare areas of wallboard along with a mostly paste-covered wall. I've sanded it with 150 grit but the walls still don't feel smooth. Do I need different technique or different tools? I know from reading various forums here that GARDZ will seal the paste so I can paint (do I need a second primer over it?).