I picked up a used bumper from a forum member and I've been getting it ready for paint. Almost done sanding it and I may start respraying it in my color tomorrow. This is my first real paint project and I noticed when I was sanding it that there wasn't much/any base coat compared to the body of the car that has several distinct layers of base coat. So my question is, is it required to spray primer coats on a urethane bumper before spraying color coats? I've read that flex additives are not required for urethane-based paint products, only for acryllics and enamels of yesteryear. However, if I do spray primer, is a flex additive required for the base coat? I have an aerosol can of adhesion promoter that I was told would help the base coat adhere to the plastic. Any insight for a first timer would be appreciated. Thanks!