I have stripped the wallpaper off of my walls- which are plaster, not drywall. Now I want to paint them. Presumably, they were prepped prior to wallpapering, or the wallpaper wouldn't have come off. My question is, do I need to prime the walls prior to painting? Should I use dywall primer? Wood/all-purpose primer? (like KILZ.) Or can I just start painting the finish color?