I haven't had too much rust inhibiting success with those "turn rust into primer" products. In all seriousness, the green bondo with pieces of fiber in it, I think it's called bondo glass or some such, works beautifully. Just sand out as much rust as possible, and the fiber bondo somehow bonds better than regular bondo to bare metal. It's made for boats so it's waterproof. One of my little fixes that I did about 1.5 years ago is still rust free.