How are you treating the rust on the back side of the panels and inaccessible locations?
It kind of depends on the location. I use a combination of products. Rust inhibitor products (paints) such as por15, eastwood rust encapsulator, rust bullet etc get used quite a lot under the car and inside these structure/support areas. I also use 2K high solids epoxy primer and in contact areas that get factory type spot welds (pinch welds) I use a zinc rich primer that can be welded through.
It pops and snaps when you start the weld but it does work and that way you have a coating on the steal in those sandwiched metal areas between and around spot welds/plug welds.
