Sand out rust... You gonna treat it also? Any rattle can paint you put over un treated rust is a complete waste of time. Hell any urethane paint you put over un treated rust is a major waste of time and money. Treat the rust.
depending on what it is, I always use a grinder with a grinding disk, or wirebrush to take heavy rust off. If it is a body, i'd use sand paper, if its anything else as in chassis , or bumpers i'd use a grinder in a heart beat.
Always the one with the most carnage, most stuck