Power Wash top and bottom
Wash with Dawn dish washing liquid with no scents or lotions (Oils)
Use a wax and grease remover
Sand to 220 grit
Reclean with Dawn and wax and grease remover
Use a flexible bumper repair for dents and scratches
Sand to 220
Clean
Use an adhesion promoter, 1 coat
Use a flex additive in all of your paint including primer
I use the Urethane paints, they seem to be more flexible