the bumper comes already primed with flexible primer from the factory ,if it's OEM bumper,I use "Scuff Stuff" paste and red scotch pad ,rinse it off, apply two wet coats of Plastic Magic adhesion promoter and follow with color and clear. Never had adhesion issues ,I love using this stuff, it works great on raw plastic bumpers too

do a google search on these products,most paint supply stores have it in stock .

whatever you do, invest in some adhesion promoter,