There seem to be two simple approaches available, Ty. Option 1, use a mechanical approach such as sand-blasting or scraping (with one or more curved scrapers). Option 2, use a de-rusting agent such as RS1. In either case there will be no impediment to rust re-starting, of course.
For many years I regarded mechanical methods as the only appropriate approach to de-rusting, but more recently I've used RS1 on a couple of repainting projects, and so far I've been happier with the results than I ever was with mechanical methods. It removes all of the rust, including deep inside lap-joints.
