It's necessary when applying fillers to TPO and polypropylene parts, like most late model bumper covers. Filler Prep allows the filler to get a good "bite" on the plastic and eliminates the peeling problems common on many automotive plastics.
Polyvance Plastic Magic Adhesion Promoter (Quart) (1050-4)
Polyvance - Plastic Magic Adhesion Promoter - 1 Quart (1050-4)
SEM 38343 Vinyl Prep Adhesion Promoter Aerosol Spray 13.3 oz
0.8 Gal Multipurpose Pro-Grade Body Filler - Superior Adhesion for All Surfaces
Polyvance Flex Filler Flexible Epoxy Adhesive/Filler, Cartridge Kit 2000-C