I’ve found epoxy to penetrate rust pretty nicely and it’s pretty impermeable; seems to exclude oxygen pretty well. Never tried it for automotive repair because it’s been awhile since I did any (I was just hacking around.)

But now I’m thinking about rat rodding an old van with big rust holes. A plan would be to just grind down the loose rust, slather on some epoxy to prime the rust and then do an ugly job of covering the holes of fiberglass cloth. (Epoxy resin is stronger that polyester (bondo et. al.) which also passes H2O and O2 readily.) Then it would get bedliner below the highlight line.

 

You could cut out the rust hole and body panel bond on a patch panel it is like epoxy, no chance of fire from welding or brazing the patch with torches!

 

Years ago I had a friend who did auto body and I had a customer that had rusted out rockers and also at the pillar and I had him do the cut and tack in new metal. The material in the wall caught on fire and he had to push it out of his barn as it went up in flames. We were compensated quickly. It's much funnier now than it was at the time.

 

We lost the barn that our car club was in back in the early 80’s , the carpet caught fire as one of the guys was welding a rust patch panel in his prized ‘66 GTO.
19 cars gone! No humans or animals were hurt. His car wasn’t insured.

 

Yep, His burnt a couple times.