Most people just sand,primer and paint nothing more and ussually one thick coat. One way I have learned to spray can stain black, is to sand, primer, sealant, FLAT BLACK (2-3 coats), wet-sand, SEMI-GLOSS CLEAR COAT (2-3 coats), wet-sand, and it will take out most of the stipes, also when you lay a coat, spray it the other way, on the 3rd coat spray it the other way again, kinda like a cross. Mainly the wet-sanding should take out the stripes. Also for poly bumper parts that are flexable, using a flexable agent in the paint works WONDERS!
TAKE YOUR TIME, its takes forever and it sucks ass. I HATE BODY WORK!