I'm getting around to repainting my hallway ceiling. It's the second floor with a cold attic above. First let me say that the paint has been up there for a while. We had painted with ceiling white over the previous owner's color years ago. For a while now, we've noticed a few small cracks in the finish, but now that I'm up there and trying to clean up those bad areas, I'm finding that pretty large areas of paint are easily chipped right off. Once I open a crack, I can push a thin scraper (putty knife) right along the edge of the paint and it separates so cleanly that the drywall looks untouched. It's easy to see that the ceiling was never primed. Of course i plan on priming this time, but I am worrying that I'm going to have trouble where the old paint still is. Is there any kind of additive that will help the old paint bond? Do I have to scrape the whole ceiling (or replace drywall)? Or should I just ignore my worries and just go forward with plan A?