Well, the totally correct way would be to prime the entire truck with a urethane primer. This would give you a good base to paint on. You could sand out this pitting with 180 or 220, and then guide coat and sand out those scratches with 320 or 400 and paint. The grit would be determaned by what you plan on painting it with. But if you didn't have a need to surface EVERY panel, you could urethane prime what is needed, and then use a sealer (be it urehtane or epoxy) and get that uniform color you are refering to.
