If you don't want to put the money into proper body work and paint, and aren't worried about the aesthetics of the quarters, there is a cheaper alternative that won't completely stop the rust but will majorly slow it down. You will need AUTOMOTIVE expanding foam (the home repair type draws moisture), 36grit sandpaper, "ONE STEP" rust converter (I don't trust the other brands), and asphault undercoat or a type of bedliner in a can. sand back past the blisters to expose all the accessible rust. knock off all the loose rust. Brush the one-step on all accessible rust and let dry. shoot just enough foam in the rust holes that is oozes back out. let the foam dry and trim it with a saw blade or box knife. spray the undercoat over all unpainted areas. DONE - ugly as sin two tone quarters that won't allow anymore moisture in.
PS. If you are any good with fiberglass, you can skip the undercoat, fiberglass patch the holes, and have a body shop paint the panel. I don't ever advise spray paint due to it's lacking a catalyst and not actually creating a proper moisture barrier.