For rust inhibiting primer, the usual Rustkill or whatever works. But it will only work as good as your preparation. Rub the area down with Deoxidine and a scourer or some steel wool to get rid of every speck of rust. No matter what you put on it, if it's not totally removed the rust will come back and the paint will just make a nice coat for the rust to grow under. I would avoid afterpay like the plague. Don't play if you can't pay.