I just repainted an OEM rear bumper 2 weeks ago...was white, painted it black... Im not aware of any way around sanding...all i did was light sanded it with 600 or 900 grit..(forget which one it was) and then clean it really well with some Laquer thinner, and it should be ready for paint...no primer needed... My bumper turned out great! It wasnt allot of work either, just scuff the old paint... remember not to touch it with your hands after its clean or you will get mad fisheyes from the oils in your skin...

Maybe somone else knowes a better way, but it worked for me...good luck!