My damn wife didn't pull the new Buick Enclave up far enough in the garage and the garage door scuffed the back bumper in about a 5"x4" section on the rear bumper in the flat paint. On my truck, I have had great results using my PC and Menzerna products on the gloss clearcoat. Would I royally screw up the flat paint on the bumper if I used the same technique on the rear bumper to remove the scratches (PC/Menzerna)? If a PC could damage the flat finish, would doing it by hand be better? The last thing I want is a glossy 5x5 spot in the center of a flat bumper! Help!