What's inside
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A butt joint puts two sheet edges in the same plane, so there is little metal available to absorb heat. On thin mild steel, the main challenge is getting enough fusion without blowing a hole or pulling the panels out of alignment. The method below suits clean steel roughly 18-gauge to 1/8-inch thick (about 1.2–3.2 mm); thinner sheet needs especially careful fit-up and short welds.
Gear and consumables
A small MIG welder with adjustable voltage and wire speed is usually the easiest choice. For steel around 18-gauge to 1/8 inch, use 0.023- or 0.030-inch solid ER70S-6 wire with a shielding gas such as 75% argon/25% carbon dioxide. The smaller wire gives you more control at low output; 0.030-inch wire is a practical all-round option if you also weld thicker material. A 0.023-inch ER70S-6 MIG wire spool can be useful if your machine feeds it reliably.
Use a sound ground clamp, clean contact tip, welding gloves, and a helmet set to a shade appropriate for your process and amperage. Wear nonflammable clothing that covers your skin, and keep a fire extinguisher nearby. Welding fumes need control: work with effective ventilation, and do not weld galvanized or coated steel until the coating has been safely removed and the fumes are controlled.
Choose the process for the sheet
| Setup | Best fit | Trade-off |
|---|---|---|
| 0.023-inch wire, 75/25 gas | Thin sheet and low-output welders | Good control, but wire feeding can be fussier |
| 0.030-inch wire, 75/25 gas | Mixed light-gauge work | Versatile, but easier to overheat very thin sheet |
| Flux-core wire | Outdoor work where wind disrupts shielding gas | More spatter and smoke; many small machines have limited low-end control |
For clean indoor sheet metal, solid wire and gas are generally easier to control and leave less cleanup. Flux-core is not automatically the cheaper choice once you account for wire, cleanup, and the extra care needed to avoid burn-through. If you need to replace or upgrade the gas setup, compare a MIG gas regulator and flowmeter that matches your cylinder valve and machine fittings.
Prepare the joint
Cut both edges straight, then remove paint, rust, oil, mill scale, and other contamination at least 1/2 inch (about 13 mm) back from the joint. A flap disc or sanding disc works; avoid thinning the edge while grinding. Degrease with a suitable cleaner and let it evaporate fully before welding. Do not use chlorinated solvents near welding heat.
Bring the edges together with a consistent gap. For sheet around 18-gauge, aim for a near-zero gap to about 1/32 inch (0.8 mm); on thicker material, a small, even gap can help fusion. A gap that varies along the seam invites burn-through where it opens up. Clamp the work flat to a steel backing bar if practical. A copper backing bar conducts heat away and will not fuse to the steel as readily, but it is optional—not a substitute for correct settings or tight fit-up.
Use clamps or magnets to hold alignment, then place small tack welds every 1–2 inches (25–50 mm). Check that the panels are still flush. If they move, correct the setup before welding the seam.
Set the welder and test
Start with the manufacturer’s chart for your wire diameter, metal thickness, and gas. Machine settings vary, so there is no dependable single voltage and wire-speed figure for all welders. Make test welds on scrap of the same thickness and joint fit. The arc should sound like a steady, crisp crackle, not a harsh bang or a soft sputter. Too much voltage or wire speed can burn through; too little heat or poor wire feeding can leave a tall bead with weak fusion at the edges.
Set gas flow near 20–25 cubic feet per hour (about 9–12 L/min) indoors, unless your equipment maker specifies otherwise. More flow is not always better: excessive flow can create turbulence and pull air into the shielding. Check that the tip is clean, the liner feeds smoothly, and the work clamp is attached to clean metal.
Weld the seam in short sections
Hold the gun at roughly a 10–15-degree push angle, with the wire aimed at the joint. Keep stick-out around 3/8 inch (10 mm), unless your gun or process calls for a different distance. Use a straight, controlled motion rather than a wide weave. On thin sheet, a narrow bead limits heat spread.
Weld in short stitches, typically 1/2–1 inch (13–25 mm) long, and skip around the joint: weld one section, move to a distant section, then fill the gaps as the panel cools. This reduces distortion compared with running one continuous bead. Let the metal cool naturally between passes; quenching hot sheet with water can increase distortion and is not a reliable fix for excess heat. Watch the puddle: if the edges begin to sag or a hole opens, stop immediately, let the area cool, and reduce heat or travel more quickly on the next attempt. Do not try to bridge a growing hole with a slow, long weld.
Inspect and correct the weld
After cooling, check that the bead ties into both edges and that the back side shows evidence of fusion where accessible. A bead that sits on top like a rope may look tidy but can be poorly fused. Porosity—small holes in the bead—often points to contamination, drafts, a gas leak, or poor shielding. Reclean and check gas coverage before adding metal. A visible crack, burn-through, or unfused edge needs repair, not just cosmetic grinding.
Grind only as much as the job requires. Grinding a thin panel flush can remove the weld and weaken the joint; use a light touch and inspect again. If the panel warps, stop grinding and welding until it cools, then correct alignment gradually with clamps or gentle mechanical work. For a structural part or any joint whose failure could cause injury, follow an appropriate qualified welding procedure rather than relying on appearance alone.