After stripping everything (suspension, fuel/brake lines etc.) I did mine with an angle grinder and wire wheels. Then I painted it with "Tremclad Rust Paint", 2 coats with a brush. I know about POR15 etc., but this didn't cost a fortune, covered the residual rust just fine and stuck like sh** to a blanket. Then I sprayed my wheel wells with Rubberized Undercoating. After 5 years, no problems - looks like it was all done yesterday and cleans up great with a wet rag. (I should mention that I only drive her on Sunny days)
Just in the middle of refinishing north of the tranny X-member while the engine is out.