[villagetinker]

Having built (OK rebuilt) 8 early Mustangs from the ground up I know what you are talking about. If this is surface rust and NOT an actual hole or real deep) you can buy at any auto parts store (1) rust converter, (2) zinc primer, and if necessary, (3) primer sealer.
If there is real deep rust (most of the metal is gone, or an actual hole, then you are into having to weld in replacement metal. Sometimes you can use a fiberglass patch, I have done both. Of course the final step is getting matching paint and the final finish.

I believe the problem you are running into is that rust is VERY hard to stop. Go to the Eastwood company website they have excellent info on this subject and I have used many of their products with excellent results.

There is a classic/antique car restoration company that I have seen advertised, if I can find or remember the name I will post another reply.