just spraying over rust with undercoating and self-etching primer won't get rid of the rust, ever if you blast hit. To do it properly, you have to remove the rusted areas, and weld on metal in it's place. Trust me. This will be the best way to do it, but even then, it won't get rid of the rust 100%, b/c if it's started in one place, chances are it's started in other places too.