i pulled a set of fenders off my 71 skylark custom and put them on my 71 skylark 4 door, the pass side fender at the very front had a some what of a medium sized pushed in dent. i was able to pull most of it. but now, after i do bondo and drive it, the stuff keeps cracking. any idea how i can stop this? this is the only thing holding me back from painting the car. well that and the back of $$ lol