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Burn-through happens when the weld puddle stays hot for too long and melts completely through the sheet. Thin steel, stainless, and aluminum have little thermal mass, so a setting that works on 1/8-inch plate can destroy 20-gauge sheet in seconds. Preventing it is mostly a matter of controlling heat, reducing the time spent in one spot, and supporting the joint properly.

Match the settings to the sheet thickness

Start with the material thickness, not the maximum output of the welder. For mild steel, 18-gauge sheet is about 0.048 inch thick, while 20-gauge is about 0.036 inch. At these thicknesses, a small increase in amperage or wire speed can change a sound weld into a hole.

Use the machine’s chart as a starting point, then test on a scrap of the same thickness. A typical short-circuit MIG setup for mild steel might use 0.023-inch solid wire with C25 shielding gas and roughly 30 to 60 amps, depending on thickness, joint fit-up, and travel speed. Do not treat those figures as universal settings: machines vary, and wire speed controls amperage on a MIG welder.

For most automotive panels and light fabrication, a 120V MIG welder set up for 0.023-inch wire is easier to control than a large machine running oversized wire. The cheaper option is fine if it has stable low-output control, proper gas regulation, and a usable duty cycle. Maximum amperage is far less important here than low-end control.

Prepare the joint before striking an arc

Fit-up often decides whether thin metal survives. A gap makes the arc bridge empty space, concentrating heat at the edges and encouraging burn-through. Aim for a tight, consistent joint. If the panels are warped, clamp them flat or use temporary sheet-metal screws, magnets, or copper backing.

Clean both sides of the joint to bright metal. Paint, rust, galvanizing, oil, and mill scale interfere with the arc and can make you slow down to compensate, adding more heat. Use a flap disc or abrasive pad carefully; grinding a thin edge too aggressively can make it even thinner. Remove coatings in a well-ventilated area, and never weld galvanized steel without proper fume controls.

Copper backing is especially useful because the molten steel does not readily fuse to copper, while the copper draws heat away. A flat copper bar behind a butt joint can prevent a hole and support the puddle. Aluminum backing can also work, but it must be clean and fit tightly. Backing is not a substitute for correct settings, but it gives you more margin for error.

Tack first, then connect the tacks

Do not run one continuous bead around a thin panel. Place small tacks roughly 1 to 2 inches apart, alternating from one side of the joint to the other. Add more tacks until the seam is held securely, then fill the spaces with short welds.

Use skip welding: make a 1/2-inch weld, move several inches away, and weld another section. Continue moving around so no area accumulates too much heat. Allow the metal to cool until it is warm rather than visibly hot before returning to an adjacent section. Compressed air can cool a repair, but avoid quenching a large stressed panel with water because rapid cooling can increase distortion.

For patch panels, a series of overlapping tacks is safer than a continuous bead. Let each tack cool enough to support the next one. If a tack turns bright orange, the puddle is too large or you are staying in place too long.

Control MIG arc length and travel speed

Use short-circuit transfer, a short stickout, and a steady travel motion. About 3/8 inch of electrode stickout is a useful starting point with small wire. Hold the gun close enough to maintain a stable arc, but do not bury the nozzle in the joint.

A slight push angle, around 10 to 15 degrees, generally gives a flatter bead and less penetration than a steep drag angle. Keep moving as soon as the puddle wets both edges. If the bead becomes wide, the edges sag, or the center turns dull and sinks, stop immediately and let the area cool.

Turn the voltage and wire speed down together when possible, then test again. Reducing only wire speed can cause an unstable, cold arc; reducing only voltage can produce excessive spatter and poor fusion. If the welder has inductance control, a little more inductance can soften the short-circuit arc, but use small adjustments and verify the result on scrap.

Use TIG when precision matters

TIG gives the best control over a small puddle, particularly on visible stainless work or thin aluminum. Use a foot pedal or fingertip amperage control if available. Set a maximum amperage appropriate for the material, then back off as the workpiece heats.

For thin steel, a sharp tungsten and a small cup help keep the arc focused. Use brief dab-and-move motions rather than feeding filler continuously into a large puddle. On some butt joints, a precise autogenous weld without filler is possible, but the joint must fit tightly and the material must be clean. Filler adds strength and fills small gaps, but it also adds heat.

For aluminum, AC TIG is normally required because the oxide layer melts at a much higher temperature than the base metal. Clean aluminum with a dedicated stainless brush, use a tight fit-up, and avoid lingering while the puddle suddenly widens. That sudden collapse is a common warning that burn-through is imminent.

Know when stick welding is the wrong tool

Stick welding is difficult on sheet metal. Even small 1/16-inch electrodes can put too much heat into 18- or 20-gauge material, and the slag makes quick starts and stops awkward. If stick is your only option, use the smallest compatible low-hydrogen or mild-steel electrode, the lowest stable amperage, short tacks, and copper backing. A larger electrode is not cheaper if it forces you to cut out holes and redo the panel.

Recognize failure before the hole forms

Common warning signs include a puddle that suddenly grows, a bead that sinks below the sheet surface, a gray or orange-hot area spreading well beyond the weld, and a sizzling arc that no longer wets both edges. Stop welding at the first sign. Let the panel cool, grind away contamination, and inspect the opposite side.

If a hole forms, do not chase it with a long bead. Place a small copper backing piece behind it and bridge the opening with short trigger taps around its edge. Allow each tap to cool before adding the next. For a badly damaged panel, cutting out the thin section and fitting a new patch is usually faster and stronger than repeatedly filling a crater.

Use protection suited to the process

Thin sheet still produces ultraviolet radiation, sparks, sharp edges, and harmful fumes. Wear a properly shaded auto-darkening variable-shade welding helmet, leather gloves, flame-resistant clothing, and hearing protection when grinding. A helmet with a clear view and reliable low-amp arc detection is worth paying for because it helps you see the puddle before it runs away.

Use local exhaust or suitable respiratory protection for coatings and stainless steel. Keep the work clamped securely, remove flammables, and inspect the back side of the panel before welding near wiring, insulation, fuel systems, or upholstery.