Rust repair on a rocker panel with a 140-amp MIG: patch panels, sheet metal and patience

Cut out everything that is thin; tap with a screwdriver handle and listen for the crunch. Make a cardboard template, cut 18 or 20 gauge patch steel, fit it with a 1/16" gap, and tack every inch, hopping around so no area gets hot. Let each tack cool to the touch before the next. Grind tacks flush, repeat until the seam is closed. Seam sealer on the back, weld-through primer where you cannot reach, epoxy primer on the front.

The mistake everyone makes is running a bead. On sheet metal you only ever stitch tacks together.