I just started working at a shop and what we do there is basically the 600 wet sanding and get everything nice and scuffed up so the paint sticks, and on bumpers, we just use the 600 dry to get any nicks out after we wet sand it, and make sure you don't make a indent concentrating on one area too much. After that, make sure you get it cleaned off a bit, with like a damp cloth and a dry one, then you can do the primer, and sand it down once or twice, but the last coat, make sure it covers everything, and i'm not totally sure you have to sand the last coat down, just ask them on that or i can tell you tommorow.