I am restoring a 1970 GTO at work right now, and getting pretty close to paint. The biggest hang up i have right now is the front "enduro" bumper. The problem is that i had someone else stip all the old paint off of it, which was a pretty big mistake, now it has a bunch of little divits in it from a da. What could i use on this that will both fill and stick to the rubber? im using Slick Sand for primer on the car, and i already know that wont work. I was thinking about using a urethane primer with flex in it for the rubber and blocking that. Does anyone have an opinion if that will work, or if there is a better method?