Thats the right idea, I'm thinking. Although I don't live in the rust belt and don't know just how rip-off prone those places that rustproof stuff are. It would take a fairly long wand, like the three footer used with 3M Rust Fighter. Perhaps you can search up some rocker rust pics folks have posted here. I can think of a couple users who have some in their car's threads. Anyway, that would give you some insight as to where the rust actually begins, inside the structure.
Using a one step rust converter on that jack point before sanding it could help. The back of the flange will need to be tended to as well.
