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Lap joints are one of the simplest MIG welding joints, but galvanized steel adds a serious complication: the zinc coating produces hazardous fumes when heated. Good results depend less on exotic equipment than on removing the coating where you weld, controlling heat, and keeping your head out of the smoke.
Before You Start: Fume Control and Safety
Welding galvanized steel can produce zinc oxide fumes that cause metal fume fever. Symptoms may include chills, headache, nausea, fever, and muscle aches several hours after exposure. Do not weld indoors without effective source capture and ventilation. A fan blowing fumes across your face is not adequate; it only moves the contamination.
Use local exhaust positioned close to the arc, while avoiding airflow strong enough to disturb the shielding gas. If the coating is extensive, work outside or use a properly ventilated welding area. Wear a welding helmet with a suitable shade, flame-resistant clothing, leather gloves, and non-vented safety glasses under the helmet. A respirator may be appropriate, but it does not replace removing zinc and providing ventilation. Follow the respirator manufacturer’s requirements for fit testing, filters, and workplace conditions.
Keep galvanized parts away from open flames and avoid heating them with a torch simply to burn the coating off. Grinding or sanding produces zinc-containing dust, so use eye protection and dust control. A dedicated welding fume extractor is a worthwhile purchase for repeated indoor work.
Choose the MIG Process and Wire
For ordinary mild-steel galvanized sheet and angle, solid ER70S-6 wire with 75 percent argon and 25 percent carbon dioxide is a practical choice. The silicon and manganese content in ER70S-6 helps tolerate small amounts of remaining surface contamination. It does not make welding directly through thick zinc safe or reliable.
Flux-core wire can be useful outdoors because it is less sensitive to wind than shielding gas. However, it creates more smoke and slag, making fume control and cleanup harder. Gas-shielded solid wire usually gives a cleaner bead and is easier to inspect. Use the wire size that suits the material and your machine:
| Material thickness | Useful wire size | Typical starting current | Notes |
|---|---|---|---|
| 1.2–2 mm sheet | 0.6 mm (0.023 in) | 45–90 A | Short-circuit transfer; use short stitches |
| 2–3 mm sheet | 0.8 mm (0.030 in) | 70–120 A | Good general-purpose setup |
| 3–6 mm plate or angle | 0.9 mm (0.035 in) | 100–170 A | More heat and slower travel may be required |
These are starting ranges, not substitutes for your welder’s chart. For a small machine, an ER70S-6 MIG wire in 0.030-inch diameter is a useful all-around option.
Prepare the Lap Joint
Cleanliness and fit-up determine whether a lap joint weld is sound. Remove oil, paint, dirt, and loose zinc first. Then grind or sand the zinc coating back at least 1/2 inch, or about 12 mm, on both sides of the planned weld. For thick galvanizing or critical work, remove more—up to 1 inch where practical. Expose bright steel along the joint, but do not thin the sheet with aggressive grinding.
Use a flap disc rather than a coarse grinding wheel on thin material. Clamp the pieces tightly so the overlapping sheets cannot lift as they heat. A lap joint should have full contact without a large gap. A gap greater than about 1/16 inch (1.5 mm) increases burn-through risk on light sheet and often creates a trapped pocket where zinc continues to vaporize.
After welding, the bare area needs corrosion protection. Zinc-rich cold galvanizing paint is convenient for general repairs, while a zinc-rich primer followed by a compatible topcoat gives better long-term protection. Do not apply coatings until the weld and surrounding steel are clean and cool.
MIG Settings and Welding Technique
Set the machine for short-circuit MIG on thin and medium steel. Start with about 20–25 cubic feet per hour (9–12 liters per minute) of gas flow, then adjust for drafts and nozzle size. Excessive flow can create turbulence and draw air into the arc. Set wire stickout around 3/8 inch (10 mm), keep the gun angle near 10–15 degrees in the direction of travel, and use a slight push technique.
Tack the joint at both ends and every 2–3 inches (50–75 mm) along a long seam. Check that the parts remain flat before making the final weld. For thin galvanized sheet, do not try to run one continuous bead. Make short 1/2- to 1-inch (12–25 mm) stitches, alternating locations to limit distortion and heat buildup. Pause long enough for the surrounding metal to cool without quenching the red-hot weld with water.
Keep the arc on the thicker member when the two pieces differ in thickness. Aim roughly 60 percent of the heat at the thicker piece and wash the puddle onto the thinner sheet. A short arc and steady travel are preferable to a wide weave. Excessive weaving increases heat input and can trap zinc vapor in the weld pool.
For lap joints, a small fillet weld with good fusion at both edges is usually stronger and cleaner than a large, slow bead. On sheet metal, a 1/8-inch (3 mm) fillet is often sufficient when the design allows it. Do not assume a larger bead compensates for poor penetration; it may simply add distortion and leave unfused metal beneath.
Inspect the Weld and Fix Problems
A sound bead should have even ripples, smooth tie-in at both edges, and no visible holes or continuous undercut. Check the back and ends of the joint for burn-through. Pinholes, worm tracks, and a gray, dirty-looking bead commonly indicate zinc contamination, poor gas coverage, excessive travel speed, or an arc that is too long.
Stop if the weld pool boils violently or produces heavy white smoke. Let the work cool, improve ventilation, and remove more coating. If the bead sits on top without fusing the lower sheet, reduce travel speed slightly, increase voltage or wire-feed settings in small steps, and verify the joint is tight. If the thin sheet burns away, lower heat, use shorter stitches, and allow more cooling time.
For structural or safety-critical work, do not rely on visual inspection alone. Galvanized coatings can hide lack of fusion and trapped contamination. Make a test joint from the same thickness and inspect a cut cross-section, or have the work evaluated according to the applicable welding code. For occasional repairs, the cheaper option—a basic MIG welder, solid wire, careful grinding, and short stitch welds—is usually enough. The upgrades that matter most are reliable ventilation, good clamps, and consistent preparation, not a premium welder.