I would sand it with some very fine grit, you don't want scratches just enough to make a good bonding surface. Then wipe it down with some acetone or a similar product to get it as clean as possible. Once it is clean and prepped just spray it down nice and even. I wouldn't necessarily try and get it all down in one thick coat but rather make it a couple of coats. Remember you can spray more on but you can't take it off if it looks all sloppy. I have done a little work like that but not a lot so maybe someone with more experience can share. Also of there are any dents and dings now would be a great time to put some bondo on and make it look professional
