Start with 400 on the spots, maybe even 600 (wet), which will either clean them up for further buffing (no paint) or you'll "burn though" and need to paint or gelcoat. I'm a big fan of not making more work then you actually need to do, so see what you have first (buff the spots) as this might be all you need, before breaking out the grinder.
If you're set on a repaint, use 120 grit all over to provide a rough tooth. Smooth this off with 150 or 180 if you like and prime with a good quality primer. Sand the primer to 220, or 280 if you're really anal, before applying top coats. As previously mentioned a good primer will easily fill up 150 grit scratches, assuming enough coats (2 minimum, 3 or more if looking to fair a little too). Most over sand the crap out of their projects (too fine a grit), thinking it makes a better finish. Modern paints will lay down and fill most sins, so let the product do it's job and stick with 220 or 280, under the topcoat, knowing then 180 will fill up just fine too. It depends on how your elbows feel about it.