I would expect to dry out the areas first. This could easily be done just by waiting for the right season and a little heat. Using a polyurethane expanding foam would eliminate air contact with the metal. No moist air, no oxidation. I'll try an experiment with some square iron tubing. At this point this is just an exercise. I'm going to do more research on Rust and see what I come up with. I'm not convinced that existing oxidation would be a reason not to do it. Not really sure it would need to be dry. Residual moisture might be neutralized by the foam. Polyurethane wood glue is actually activated by moistening one side of the glued surface with water.
Ron