Sevenson, The most important thing you can do is to use metal prep on the pitted areas before you prime. It contains Phosphoric acid, which changes the rust from iron oxide (which is active), to iron phosphate (which just lays there forever).

Another option is to sand it as well as possible, then use a self-etching primer (which contains phosphoric acid). Jay

Any body shop can help you with advice on how to use these products.