Three kinds of rust, three different repairs
Surface rust is cosmetic. It hasn't pitted the metal yet and can be sanded, treated, and repainted. Penetrating rust has eaten through the surface layer and usually means the affected section needs to be cut out and welded-in with a new patch. Structural rust (rust in frame rails, rocker panels, or unibody sections) is a safety issue. Structural rust can usually be fixed, but only with proper cutout, reinforcement, and welding to OEM procedures.