Direct answer: welded panel replacement succeeds on four techniques — drill out factory spot welds cleanly, trial-fit with clamps before any weld, plug-weld at original locations with weld-through primer on the flanges, and control heat by welding in short, scattered sequences. Master those and a MIG welder, and structural panels are within reach; skip any one and the repair announces itself forever.
Tools that matter
A MIG welder (gas, not flux-core, for sheet metal), spot-weld drill bits, panel clamps and self-tapping screws, weld-through primer, flap discs, hammer and dolly, seam sealer, and patience measured in hours not minutes.
Removing the old panel
Find every factory spot weld (a wire wheel reveals them), centre-punch, and drill through the top layer only. Chisel the panel free, then dress the remaining flange flat. Cutting corners here — literally, with an angle grinder through both layers — destroys the flange the new panel needs.
Trial fit: the step everyone skips
Clamp the new pressing in place. Hang the door, tailgate or hood it gaps against. Stand back, check swage line alignment, measure the gaps, adjust. Do this three times before the first weld — because after the first weld, adjustment is surgery. This is also when you confirm the panel itself (a correct-profile pressing clamps up sweetly; fighting the clamps means stop and reassess).
Plug welding
Punch or drill 6–8 mm holes in the new panel at the factory spot-weld spacing, coat mating flanges with weld-through primer, clamp, and fill each hole with a weld that fuses to the panel beneath. Work scattered — never sequential — so heat can't accumulate and pull the panel.
Butt welding (patch sections)
For patch panels: cut the rust out with a clean margin, match the patch edge-to-edge with zero gap or overlap, tack every 25 mm around the joint, then fill between tacks in one-second bursts, moving around the panel. Dress with a flap disc, planish with hammer and dolly, skim with filler. Lap joints with sealer bubble within two winters — butt-weld or don't bother.
Heat is the enemy
Sheet steel warps at temperatures a MIG reaches instantly. Short bursts, scattered sequence, compressed air between welds if you must — but never a continuous bead along a panel joint, and never chase a warp with more weld.
Sealing the repair
Weld-through primer inside the joint, dressed welds outside, epoxy primer, factory-pattern seam sealer, cavity wax in every enclosed section. The factory seams rusted because they were bare inside; yours won't be.
Structural pressings to practise on: floors · quarters · wheel housings