I understand what you are talking about . I work in a garage and have ran into the same problems myself . These are some of the things I do / try
wet sand and re-prep your bumper . If you are useing a sealer ( non sanding ) , apply in mist coats . small sections of the bumper at a time . Blow drying the sealer ( I use my paint gun for this ) on as you apply . You can also use a heat gun as to dry the paint on faster . As you get the sealer to stick , go over it a few times in real light coats . Let the sealer dry well , 30 minutes or so . check and make sure your surface is smooth . If it is run a tack rag over the bumper and then apply your color . If it has some ruffness then lightly sand and mist more sealer over just the bad spots . SOme of these parts can be a real pain in the butt ! Good luck .
I was Punxsy Stang before !