MIG Welding in Collision Repair
MIG (Metal Inert Gas) welding — technically GMAW (Gas Metal Arc Welding) — is the primary welding process for joining mild steel and high-strength steel body panels in collision repair. The welder feeds a consumable wire electrode through a torch, creating an arc that melts both the wire and the base metal. Shielding gas protects the molten weld pool from atmospheric contamination. The result, when done correctly, is a joint as strong as the surrounding base metal.
Body panel welding differs from structural steel welding because the material is thin (0.6–1.0mm for outer skins, 1.0–2.0mm for inner structures), heat-sensitive (distortion occurs at much lower heat input than on thick plate), and cosmetic (weld crowns must be ground flush for paint). The settings and technique that work on 1/4-inch structural steel will blow through a 0.8mm door skin in half a second.
Machine Setup
Wire Selection
Use ER70S-6 wire in 0.023-inch (0.6mm) diameter for body panel welding. This is the standard wire for thin-gauge automotive steel. The 0.023 diameter provides a small, controllable arc on thin material — larger wire (0.030 or 0.035) delivers too much heat per unit time and increases burn-through risk. ER70S-6 has deoxidizers that improve weld quality on lightly contaminated surfaces — important in collision repair where perfect cleaning isn't always achievable.
Shielding Gas
Use 75% argon / 25% CO2 (C25 mix) for mild and high-strength steel body panels. This mix provides good arc stability, moderate penetration, and minimal spatter on thin material. Some shops run straight CO2 for cost savings — it works but produces more spatter, a harsher arc, and deeper penetration that increases burn-through risk on thin panels. The cost difference between C25 and straight CO2 is minimal per tank — the quality improvement justifies the upgrade.
Gas flow rate: 25–30 CFH (cubic feet per hour) at the torch. Too little gas allows atmospheric contamination (porosity in the weld). Too much gas creates turbulence at the nozzle that draws in ambient air — achieving the same contamination result as too little gas.
Voltage and Wire Speed
For 0.8mm body panel steel with 0.023 wire on C25 gas, start with these baseline settings and adjust from there:
| Parameter | Baseline Setting | Adjustment |
|---|---|---|
| Voltage | 17–18 volts | Lower for thinner material, higher for thicker |
| Wire feed speed | 180–220 IPM | Lower for less heat, higher for more penetration |
| Wire diameter | 0.023 inch (0.6mm) | Standard for body panels |
These settings produce a short-circuit transfer arc — the wire touches the workpiece, shorts, melts a small pool, and repeats at high frequency. Short-circuit transfer is the appropriate mode for thin panel welding because it delivers minimal heat input per weld cycle. Spray transfer (higher voltage and wire speed) delivers too much heat for thin panels.
Plug Weld Technique
Plug welds are the most common weld type in collision repair panel replacement. You drill a hole through the outer panel, clamp it to the inner structure, and fill the hole with weld, fusing the outer panel to the inner panel through the plug. The result replicates the factory spot weld that originally joined the panels.
Hole Preparation
Drill or punch 8mm (5/16-inch) holes in the replacement outer panel at the locations specified by the OEM repair procedure. Space holes 25–40mm apart (approximately 1–1.5 inches) along the flange, matching the factory spot weld pattern. Use a spot weld drill bit or a dedicated panel punch — standard twist drills create rough holes that don't fill cleanly.
Welding the Plug
Position the torch perpendicular to the panel, centered over the hole. Start the arc on the inner panel visible through the hole — not on the edge of the hole itself. Starting on the hole edge melts the thin outer panel before the weld pool forms, creating a blow-out instead of a plug.
Hold the arc on the inner panel for 1–2 seconds until you see the weld pool form, then slowly move in a tight circle to fill the hole from the bottom up. The weld pool rises through the hole, fusing the outer panel edges as it fills. When the hole is full and the weld crown sits slightly above the outer panel surface, release the trigger. Total weld time per plug: 2–4 seconds.
Heat Management
Never weld consecutive plugs in sequence. Weld plug #1, skip two holes, weld plug #4, skip two more, weld plug #7 — then come back and fill the gaps. This staggered pattern distributes heat across the flange instead of concentrating it in one area. Concentrated heat warps the flange, pulls the panel, and creates distortion that requires hours of hammer-and-dolly or filler work to correct.
Between plugs, allow 15–30 seconds of cooling. Touch the panel 2 inches from the weld with a gloved hand — if it's too hot to touch comfortably, wait longer. Some techs use a damp cloth to accelerate cooling, but water on hot steel can cause hydrogen embrittlement on high-strength steels. Let air cool naturally unless the OEM procedure specifically permits wet cooling.
Stitch Welding
Stitch welds are short, intermittent MIG beads used where plug welds aren't appropriate — butt joints, open seams, and repair sections where the panel edges meet without a lap flange. Weld a 1/2-inch bead, stop, skip 1 inch, weld another 1/2-inch bead, repeat. The stitching pattern manages heat the same way staggered plug welds do — distributing heat input to prevent distortion.
Allow each stitch to cool before welding the adjacent gap. Once all stitches are complete and cooled, fill the gaps between stitches if a continuous weld is required. This two-pass approach (stitch, then fill) produces a continuous weld with less total heat distortion than a single continuous bead.
Weld Finishing
Grind plug weld crowns flush with the panel surface using a 40–80-grit flap disc on an angle grinder. Use light pressure and multiple passes — aggressive grinding thins the base metal around the weld, weakening the joint and creating a depression that needs filler. The finished weld should be flush with the panel surface with no crown, no undercut, and no visible heat discoloration beyond the immediate weld zone.
After grinding, apply epoxy primer over the weld area immediately. The heat-affected zone is bare metal with no corrosion protection — flash rust starts within hours. Two coats of epoxy primer seal the weld and surrounding metal before the repair progresses to filler or primer-surfacer.
Common Body Panel Welding Mistakes
Burn-through: Voltage too high, wire speed too fast, or torch held in one spot too long. Reduce voltage by 1 volt, reduce wire speed by 20 IPM, and keep the torch moving. If burn-through occurs, grind the area clean and re-weld with adjusted settings — a blown-through hole that's been welded over without cleaning contains porosity that weakens the joint.
Cold weld (no fusion): The weld bead sits on top of the base metal without melting into it. Caused by voltage too low, wire speed too slow, or contaminated base metal. Grind the cold weld off completely, clean the base metal to bright steel, and re-weld with slightly higher settings.
Excessive heat distortion: Welding too many consecutive plugs, using too-high settings, or not allowing cooling between welds. Once distortion occurs, it's difficult to correct without hammer-and-dolly work. Prevention through staggered welding patterns and adequate cooling is the only practical approach.
Porosity (holes in the weld): Caused by insufficient shielding gas (check flow rate and hose for leaks), contaminated base metal (paint, primer, oil, or moisture in the weld zone), or wind disrupting the gas shield (keep booth doors closed during welding). Porous welds are weak — grind out the porous section and re-weld over clean metal.
Back to Pro Tips
