Re: Cracks in Paint............

I'm not a painter, I'm a carpenter. I've done supervision on a few remodels.
Painting is 90% prep. Unless it's newly taped and floated drywall or plaster you need to know what you're covering. If you are painting over existing paint or wall paper like the other poster's said it has to be stable. In every project I've been on if it was paint over wall paper we removed the wall paper. It's a PITA from what I've observed. I think you can paint over it but I'm not sure how they prep it or what kind of papering can be painted over.
I guess what I was getting at is you need to spend time scraping, removing and sanding old paint. If you have a wall in good condition, no cracking, bubbling, pealing you likely can paint over it without much preparation other than TSP.