I used duplicolor bedliner:
scuff up your bumper, not aggressive
clean it off with some acetone or similar, use white cloth you'll be able to see when the bumper is clean
After it's clean go over it with some tack cloth
hang it where you can reach it easily to paint
spray with 2-3 coats of self etching primer, don't apply heavy coats, use several light coats - let it dry between coats
spray with bedliner on or your paint choice, several light coats- let it dry between coats.
do good prep work and your paint job will stick better.