POR-15 seems to do a very good job of stopping the rust from coming back thru. We used it as a spot primer on my son's '90 Wrangler when we repainted it two years ago. Grind/wire wheel/sand as best you can, paint with a disposable brush (this stuff does not clean up - period!) and let dry. It apparently chemically bonds to rust. You can put Bondo over it and you can sand it (but it is very hard).
So far, only one small rust thru by the one hood latch.
Search the forum and you'll find lots of people have used it. Brace yourself for the price though - it was ~$50/quart at our local NAPA Auto Parts store.
PS - We sprayed the Jeep in a garage. Not perfect but not bad.