I by no means am condoning this venture. That being said, if you were paying me to do this, I would sand the bumpers with 80 grit with a dual action sander, spray it with Bulldog adhesion promoter, polyester prime them, sand that smooth and paint them and hope it sticks.
The grill is plastic. 80 grit would pretty much ruin it. Id probably go lightly with 150 or 220 and stay away from the edges too much. then repeat the procedure previously stated.
