peeling and sand scratches
If you sand with 320 grit and move to 600 you are leaving yourself open for sand scratch problems, especially with water born. Water born generally dries thinner than solvent. Think of it this way, 320 git paper leaves deep sand scratch in relationship to the finer 600 grit. Unless you are really thorough (and spend more time than if you used an intermediate grit, you won't sand the tops off of the 320 grit scratch. Rule of thumb, never jump more than 200 git in any final sanding project. Start with 400 grit and then move to your finer paper. The deeper the scratch or the courser the grit the smaller the jump in your next grit size. Un-primed filler 40-80-180 (and finer if you like). Primmer, 400-600-whatever, colour sanding, (depending on how well the clear laid down, if you have allot of orange peel 800 -1200- 1500-2000 and even 2500.