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A lap joint is one of the most useful MIG welds for thin steel: one piece overlaps another, and the weld joins the exposed edge of the upper piece to the lower sheet. It is common in automotive repair, brackets, ducting, patch panels, and light fabrication. The main challenge is controlling heat. Thin steel burns through quickly, distorts easily, and can look welded while remaining weak or porous underneath.
Equipment and materials
Use a MIG welder capable of short-circuit welding at low amperage. For occasional repairs, a compact 120-volt machine is often enough for 18- to 20-gauge steel. A 240-volt welder gives you more reserve, but it is not automatically better for sheet metal; excessive minimum output can make thin steel harder to weld.
For most thin mild-steel lap joints, use solid ER70S-6 wire with 75% argon and 25% carbon dioxide shielding gas. A .023-inch MIG wire is useful on very thin sheet, while .030-inch wire is a practical all-around choice for roughly 18-gauge steel and thicker material. Use direct current electrode positive (DCEP), which is the normal polarity for solid MIG wire.
You will also need a clean nozzle, contact tip matched to the wire, a wire brush, an angle grinder or flap disc, clamps, and a welding helmet. Choose an auto-darkening helmet with a clear lens and a shade around 10 for MIG work. Wear a flame-resistant jacket, gloves, safety glasses, hearing protection when grinding, and closed leather footwear. A quality auto-darkening MIG helmet is worth buying; a poor lens makes it harder to see the puddle and increases the chance of a bad weld.
Starting settings for thin steel
Welder charts are a starting point, not a guarantee. Machines vary, and joint fit-up changes the required heat. Set the gas flow around 20 cubic feet per hour indoors, then adjust if drafts disturb the shielding. Too much gas can create turbulence and pull air into the weld.
| Steel thickness | Wire size | Starting voltage | Starting wire speed | Technique |
|---|---|---|---|---|
| 22 gauge, about 0.030 in. | .023 in. | About 14–15 V | About 180–260 ipm | Short tacks or quick stitches |
| 20 gauge, about 0.036 in. | .023 or .030 in. | About 15–16 V | About 200–300 ipm | Stitch welding, moderate travel |
| 18 gauge, about 0.048 in. | .030 in. | About 16–17 V | About 220–330 ipm | Short continuous sections |
Use these numbers only as a test range. The arc should sound like a steady frying or sizzling noise. A loud, irregular crackle often means the settings or wire stickout are wrong. Excessive wire speed can push the gun away from the work, while too little wire speed creates a long, unstable arc. Keep stickout near 3/8 inch for short-circuit MIG.
Prepare the lap joint
Cut the sheets square and overlap them by about 1/2 to 1 inch. More overlap adds weight and can trap moisture, while too little overlap leaves a narrow, difficult weld. Clean at least 1 inch on both sides of the joint down to bright metal. Remove paint, rust, oil, mill scale, and especially galvanized coating. Welding zinc produces hazardous fumes; do not weld galvanized steel without proper removal, ventilation, and respiratory controls.
Keep the joint tight. A gap increases the chance of burn-through, although a tiny gap may be unavoidable on a warped panel. Clamp the pieces firmly and place tack welds every 2 to 3 inches. Check alignment after each tack because heat can pull the upper sheet out of position. For long panels, alternate tack locations rather than welding from one end to the other.
Welding the lap joint
Point the gun at the exposed edge of the upper sheet, but angle it slightly toward the lower sheet. A work angle of roughly 45 degrees between the two pieces helps distribute heat into both layers. Use a small travel angle of about 5 to 15 degrees in the direction of travel. A slight push angle usually gives a flatter bead and better visibility with short-circuit MIG.
Start the arc on the upper edge and move steadily into the lower sheet. The puddle should wet both pieces without falling through the upper edge. Do not weave on thin steel. A straight bead with a small circular motion, if any, gives better heat control than a wide side-to-side pattern.
For sheet thinner than about 18 gauge, use a stitch or skip-welding method. Weld a 1/2-inch section, stop, and move to a cool area. Continue in alternating locations until the joint is filled. Let the panel cool between passes; forced air can help, but avoid quenching hot steel with water because it can distort the panel and affect the metal.
If you are learning, practice on identical scrap with the same overlap. Make several test welds, then bend or cut them apart. A bead sitting on top like a rope indicates poor fusion. A narrow bead with a visible dark line along one edge often means the arc did not wet the lower sheet. A hole means too much heat, too slow a travel speed, too large a gap, or an overly long weld.
Fix common failures
Burn-through is usually solved by increasing travel speed, lowering voltage or wire speed, using smaller wire, and shortening each stitch. If the joint has a gap, close it with clamps or copper backing rather than trying to bridge it with a long hot bead.
Porosity points to contaminated steel, poor gas coverage, a leaking hose, a blocked nozzle, or drafts. Clean the steel again, keep the nozzle about 3/8 inch from the work, and check that the gas flows when the trigger is pulled. Do not weld outside in wind without effective shielding; even a light breeze can ruin the gas envelope.
Distortion comes from uneven heating. Use more tacks, shorter welds, alternating sides, and clamps. A continuous bead may look neater, but on thin sheet it commonly creates a warped panel. For low-cost work such as a non-structural cover, an inexpensive 120-volt MIG welder and .030-inch wire are fine. Spend more on the machine only when you need smoother low-amperage control, a longer duty cycle, or frequent fabrication.
Finish and inspect the weld
Brush off spatter and inspect both edges. The bead should be continuous where intended, fused into both sheets, and free of pinholes, cracks, undercut, and visible gaps. Grind it flush only when appearance or clearance requires it; excessive grinding can remove the weld’s useful throat. If the joint will see vibration or load, add enough weld length for the application rather than relying on a decorative bead. Seal, prime, and paint the cleaned joint to prevent corrosion between the overlapping sheets.