Repaired Quarter Panel Rust Issue

[dhutton]

I don't think it's rust, the body filler is swelling due to moisture wicking through porous welds (pin holes). Did you use butt welds? If you did you need to fill in any voids and then coat with epoxy before you apply filler. Body filler absorbs moisture and then it swells. It needs to be fully sealed from moisture on both sides. Applying body filler to bare metal is bad practice in my opinion.