Rust Treatment

I've got a 63 Dart, and the bottom of my passenger side doors are rusting through. I work at an Auto Parts store and what's available through my store is Mar-Hyde "One Step Converts rust to black primer sealer"... Says it goes on clear and only turns black on rust. Also: Napa/Permatex brand Rust Treatment, "Destroy's Rust" which you apply with a paint brush, will stain other surfaces, and Napa/Permatex brand "Rust Dissolver" which is also paint brush applied. I think the spray can of Mar-Hyde is my best choice. Any input? Should I venture to another store to get something more quality? Any help would be greatly appreciated.

Thanks!
Ashley