If you put a good epoxy based primer on bare metal, you can submerge the car in a lake and pull it a year from now and it will not be rusty. But you must get any area that is bare covered in the epoxy . I have been using PPG 50dp and 40 dp hardener for years and have never had a rust problem. The other nice thing about using epoxy based primer is that body work can be done over it with a light scratching with some 80 grit paper for better bond.