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The rear 1/4 of my 83 Bronco's frame is quite rusty due to road salt. That part of the frame is quite pitted but no holes. How much rust is acceptable here.
I would get under there and sand it, you won't know exactly how much metal is eaten away untill you knock all the rust off, then you can paint the frame with rustoleum paint or undercoat it.
Cover all the rust with some used motor oil. It will keep it from rusting. Spray some in yours as well.
How much rust is to much? I fixed a truck for a co-worker and this F150 has some bad frame rot. The rear leaf spring pearches had rusted off. There is even a few hole in the frame but he still drives it now that the rear springs are attached to something solid. It makes a good work truck for him.