Saw an episode of Musclecar a while back where the guy said the best process he had found was to scrub the bumper down with red scotchbrite, (I'd use Comet with it to clean everything), sprayed adhesion promoter then epoxy primer then the paint. He used all HOK products, sure they payed for the advertising that day, but there's lots of AP's out there, Bulldog is another and you can get it at walmart. If you want to go straight to paint with any block sanding that would probably be a good process, if you want to block the primer out for a little flatter finish I'd blast it then put epoxy down followed buy some high build primer then block that before paint. I've had lifting issues with Bulldog in the past if you try to reaply something over a break thru the featheredge will want to lift from the solvents in the paint or primer. I wouldn't recomend using the AP if you plan on blocking it flat for that reason.