Main thing to remember here that the act of rusting is a chemical process, and to reverse it you must use chemicals. Remember safety first, use good gloves and face/eye/lung protection. Just because you have sanded the rust away and it looks gone it will come back if not treated properly. Anyway sand down to bare metal, I like those 3-m fibre wheels you can attach to your drill they work good. Then treat it with an etcher, I use muriatic acid you can buy it in the paint section at any hardware store. Or you can talk to your autobody supply store and see what they recommend. then paint it with a rust inhibiting primer. Also remember that most primers have no sealer and will let moisture and air at the metal and your rust will reappear if you don't paint it in a timely fashion. Good luck.