I used Rustolium rust reformer on my 90s heavily rusted frame after i pounded the hell outa it to get the flakey scale off, and then use a wire wheel to get the dusty crap off. Frame is pitted badly in the rear and i had to cut out and weld in new sections. I put 3-4 coats probly more of rust reformer on the frame back there. Its been like 7 months now and its been wet even coated in road chemicals from the winter. And not a spec of rust is coming through. Same with the floorboards under the jeep passenger and drivers side. Its holding pretty well. Just use MANY coats and its extrremely important to clean it of the scaley flakey rust.
I doubt your frame can be much worse then mine.. in this picture. I dont have a pic of it painted but im sure you get the idea of what i had to do to fix my frame rails and leaf spring mounts in the back. TBH it took me only two days to fix my frame, using a $100 harbor freight flux welder, and like 4 cans of rust reformer. Just dont breathe in the fumes.. it puts out alot of foggy fumes.. get a respirator for chemicals.. (just a regular p100 mask you need one for chemicals) or hold your breathe.. spray and roll away to take a breath and repeat.. you dont want cancer or neurological issues.
Dont get an epoxy paint.. You want something that CONVERTS rust.. So use a rust reformer and then if you want added assurance get POR15 to 100% seal it from air.. POR15 doesnt convert rust though so thats why i tell people to not use it directly over the rust.. Use a rust reformer first. Harbor freight sell rustolium rust reformer cans for $5 or so. Get the rustolium brand dont go cheap and get the off-brand