Additionally, before any skim coat is applied to your body work, feather the edges into the old paint with at least 150 grit paper, and do not prime over 40 grit scratches. They will haunt you later. If you can feel the scratch with your fingernail, then buzz it down a little more with a d.a. More importantly, any low spots that aren't scratched up with the long block should be scuffed by hand. No natural bondo spots. These can show up later as humps in your new shiny paint.