Why Is My MIG Weld Burning Through Sheet Metal?

A MIG weld burns through sheet metal when the arc and puddle put heat into the joint faster than the thin base metal can carry it away. The usual causes are settings that are too hot, slow travel, a long arc, wide gaps, repeated welding in one area, or metal that has been ground thinner than expected. Dirty metal can also make the arc less stable and lead you to dwell too long.

Key Causes of Burn-Through in MIG Welding

Burn-through means the weld penetrates completely through the base metal and leaves a hole or a sagging area. It is especially common on thin sheet because there is little metal available to absorb and spread the arc’s heat. Miller identifies excessive heat as the main cause and recommends reducing voltage or wire feed speed and increasing travel speed when appropriate.

What You See	Likely Cause	First Correction
The puddle suddenly drops through	Voltage or wire feed is too high, travel is too slow, or the joint is already heat-soaked	Stop, let the panel cool, and reduce heat in small steps
The hole starts at a joint gap	Poor fit-up leaves no metal bridge to absorb heat	Re-clamp the parts, close the gap, or use a backing bar
The bead becomes wide and flat before failure	Travel is too slow or the arc remains in one place too long	Move faster in a straight line and keep the puddle small
Only a ground or repaired area burns through	The sheet is thinner there than the machine setting assumes	Treat the thinnest section as the true material thickness
The arc is erratic and the puddle is hard to read	Contamination, poor work-clamp contact, inconsistent wire feeding, or excessive stick-out	Clean the joint and clamp point, then inspect the gun and wire feed

Slow travel speed makes the problem worse because the arc stays over one area and builds a wide, overheated puddle. Excessive weaving has the same effect. Poor joint fit-up is another major cause: even a small open gap in very thin steel removes the heat sink that the surrounding metal would normally provide.

Dirty metal is usually an indirect cause rather than the main source of burn-through. Oil, paint, rust, mill scale, or a weak work-clamp connection can make the arc unstable, create porosity, and tempt you to pause over the joint. Over-grinding is more direct because it physically reduces the sheet thickness. Proper metal preparation techniques can help, but use settings and polarity that match the actual process rather than copying flux-core values to solid-wire MIG.

The goal on thin sheet is not the coldest possible arc. It is the lowest stable setting that still produces fusion while keeping the puddle small and moving.

The Importance of Material Preparation and Cleaning

Good material preparation improves arc stability and makes heat easier to control. Remove oil, grease, paint, rust, and heavy mill scale from the weld zone. Clean the location for the work clamp as well. A wire brush, scraper, grinder, or flap disc may be appropriate, but avoid thinning the sheet while you clean it.

Clean both sides of the joint when possible. Coatings on the back can still burn and create fumes even when the front looks clean. For mild steel, proper cleaning reduces porosity and helps you see the puddle more clearly. Aluminum and stainless steel require material-specific cleaning tools and filler choices; do not assume mild-steel settings apply to them.

A tight joint fit-up is equally important. Clamp the parts so the edges touch consistently, and place tacks close enough to hold alignment without forcing a long continuous bead. Check repaired or ground areas with extra care because they may be thinner than the surrounding panel.

Effective Techniques to Prevent Burn-Through

Preventing burn-through requires several small controls working together. Start with the correct process setup, then limit how long the arc stays over any one area. Do not try to solve every problem by turning the machine down until the bead merely sits on top; a weld that looks neat but lacks fusion is not a successful repair.

Adjusting Welder Settings

Begin with the chart inside the welder, the owner’s manual, or the wire manufacturer’s data for the exact material thickness, wire diameter, and shielding gas. On many constant-voltage MIG machines, increasing wire feed speed raises amperage and deposition, while voltage changes arc length and bead shape. Adjust the two controls in small steps so the arc remains crisp and stable.

For thin mild steel, a .023- or .025-inch solid wire often operates more comfortably at lower current than larger wire. Miller lists .023-inch wire for roughly 30-130 amps as a general selection range, but the machine’s own chart takes priority. Install the contact tip and drive-roll groove that match the wire size.

Short-circuit transfer is commonly used for thin steel. Keep the contact-tip-to-work distance consistent; about 3/8 inch is a common short-circuit target, while manufacturer guidance may allow a wider range. The original fixed 1/2-inch rule is therefore better treated as an upper-end reference rather than a universal setting.

Use a smaller wire diameter and systematic troubleshooting when the welder’s approved chart supports it. If the arc stubs into the work, the voltage may be too low for the selected wire feed. If the arc becomes harsh, erratic, or washes the puddle too wide, the voltage may be too high. Change one variable at a time and record the result.

Optimizing Travel Speed and Gun Technique

Travel speed controls how long the arc heats each section of the joint. Moving too slowly creates a wide puddle and can cause burn-through. Moving too fast can produce a narrow bead with poor tie-in or lack of fusion. Use the fastest straight-line travel that still keeps the bead tied into both edges.

1. Use a small push angle: A 5- to 15-degree push angle generally gives good visibility and can reduce penetration compared with dragging the gun.
2. Keep the arc on the leading edge: Do not let a large puddle run ahead of the wire.
3. Avoid wide weaving: Side-to-side motion increases dwell time and heat on thin sheet.
4. Watch puddle size: If it becomes wider than the thin section you are joining or begins to sag, stop and let the metal cool.
5. Keep stick-out consistent: Large changes in contact-tip-to-work distance change current and shielding behavior.

Using Tack, Skip, and Stitch Welding

Short welds help only when they are placed in a sequence that spreads heat. Repeatedly stacking hot tacks in the same area can still blow a hole.

1. Place small tacks at several points to lock the joint in alignment.
2. Weld a short section at one end, then move to a distant, cooler area.
3. Alternate sides or locations instead of progressing continuously along the seam.
4. Allow enough cooling time that the next tack does not instantly remelt a large soft area.
5. Connect the short sections only as much as the joint design requires.

On non-structural sheet-metal seams, skip or stitch welding also reduces warping. A continuous bead is not automatically better; the required weld length and spacing should follow the design, repair procedure, or applicable welding specification.

Using Tight Fit-Up and a Backing Bar

Close gaps before welding whenever possible. If a gap cannot be eliminated, reduce it with careful fitting rather than trying to bridge it with a large, hot puddle. Clamp the panel firmly so it cannot spring apart as it heats.

A copper or aluminum backing bar can be clamped tightly behind the joint. The bar supports the molten puddle and conducts heat away from the sheet. Copper is popular because steel weld metal does not readily fuse to it under normal sheet-metal MIG conditions. Keep the backing surface clean and in full contact with the work.

Best MIG Settings for Thin Metal

There is no single voltage and wire-speed setting that is correct for every 18-, 20-, or 22-gauge panel. Output varies by welder design, input power, wire chemistry, gas, polarity, joint type, position, contact-tip-to-work distance, and actual sheet thickness. Use the manufacturer’s chart as the starting point, then tune on matching scrap.

Variable	Practical Starting Approach	Signs It Needs Correction
Wire diameter	Use .023 or .025 inch for thin mild steel when approved by the welder and filler manufacturer	Larger wire may be difficult to run smoothly at the low output the sheet needs
Voltage and wire feed	Start at the chart value or low end of the approved range; reduce in small matched steps if the puddle is too hot	Too hot: wide puddle, undercut, sagging, or a hole. Too cold: stubbing, a tall rope-like bead, or poor toe fusion
Transfer mode	Use short-circuit transfer unless the machine’s approved sheet-metal or pulsed program specifies otherwise	An unsuitable high-energy transfer mode can overheat thin sheet quickly
Polarity	Follow the wire label and machine manual; most gas-shielded solid MIG wire uses electrode-positive polarity	Wrong polarity can cause an unstable arc, spatter, and poor penetration
Shielding gas	Use the gas recommended for the wire and material; for short-circuit MIG, Miller gives 25-35 cfh as a general flow range	Too little flow or drafts cause poor coverage; excessive flow can create turbulence and porosity
Contact-tip-to-work distance	Keep it steady, commonly near 3/8 inch for short-circuit transfer, unless the manufacturer specifies otherwise	Excessive or changing distance alters current, arc behavior, and gas coverage

For 18-gauge sheet metal, begin with the welder’s 18-gauge setting rather than a generic number copied from another machine. Make a one-second tack on scrap, inspect the front and back, and then extend the weld only after the tack shows fusion without a sagging center.

Material matters. Mild-steel guidance does not automatically apply to aluminum or stainless steel. Aluminum usually needs different wire-feeding equipment and gas, while stainless requires the correct filler and shielding-gas recommendation. Use material-specific guidance such as the correct voltage and wire speed for 304 stainless steel rather than reusing a mild-steel setup.

Using a MIG wire-speed and voltage chart is useful only when the chart matches the machine, wire, gas, and metal thickness. Fine-tune from that starting point by reading the puddle and inspecting the bead.

How to Correct Burn-Through Issues

When a hole forms, stop welding. Continuing to pour wire into an overheated opening usually makes the hole larger. Let the panel cool, identify the cause, and correct the setup before attempting the repair.

1. Cool and inspect the area: Check whether the hole started at a gap, a thin ground spot, an edge, or an overheated tack.
2. Clean back to sound metal: Remove loose, contaminated, or oxidized edges without enlarging the hole more than necessary.
3. Clamp a backing plate behind it: Hold clean copper tightly against the back when access allows. It supports the puddle and absorbs heat.
4. Reduce heat input: Lower voltage or wire feed speed in small coordinated steps and confirm a stable arc on scrap.
5. Start at the edge of sound metal: Place a very short tack that bridges only a small part of the opening.
6. Move to the opposite side: Alternate around the hole so no single edge becomes heat-soaked.
7. Build inward with cooling pauses: Add small tacks until the opening closes, then allow the repair to cool naturally.
8. Dress and inspect the repair: Grind only enough to restore the required profile. Check both sides for cracks, pinholes, lack of fusion, and excessive thinning.

Increasing travel speed also helps when the original failure came from dwelling too long. A copper backing plate is especially useful, but it cannot compensate for settings that are far too hot or a gap that is too wide.

Do not copy a flux-core amperage chart directly to solid-wire MIG. Flux-cored wire may use different polarity, wire diameter, transfer behavior, and voltage ranges. Always return to the chart for the process actually being used.

Common MIG Welding Issues and Their Solutions

Burn-through can appear alongside other MIG defects. Correctly identifying the symptom prevents you from making the wrong adjustment.

Problem	Typical Clues	What to Check
Burn-through	Hole, sagging puddle, or full penetration through the sheet	Heat, travel speed, fit-up, metal thickness, and cooling sequence
Lack of fusion or cold lap	Tall bead sitting on the surface with poor tie-in at the edges	Settings may be too cold, travel may be too fast, or the gun angle may be wrong
Porosity	Visible pinholes or gas pockets	Dirty metal, drafts, gas leaks, blocked nozzle, wrong flow, or excessive stick-out
Excessive spatter	Heavy droplets around the bead and an unstable sound	Voltage-to-wire-feed balance, polarity, work clamp, consumables, contamination, and gas
Warping or distortion	Panel pulls, buckles, or changes gap as welding continues	Tack spacing, clamp rigidity, weld sequence, bead length, and total heat input

A smooth, narrow bead with clear tie-in is a better sign than bead appearance alone, but visual inspection cannot prove the internal quality of a critical weld. When strength matters, follow the required inspection and test method for the job.

What to Check Before You Start Welding Thin Sheet Metal

Use this checklist before striking the arc. It catches many causes of burn-through before they damage the final part.

• Confirm the base metal type and measure or verify its actual thickness.
• Check for hidden coatings, seam sealer, insulation, fuel, wiring, or combustible material on the back side.
• Match the wire diameter, contact tip, drive roll, polarity, and shielding gas to the machine and filler-metal instructions.
• Clean both sides of the joint and the work-clamp contact point when possible.
• Clamp the joint tightly and correct gaps before welding.
• Set voltage and wire feed from the welder’s chart rather than from a different machine.
• Keep the gun angle and contact-tip-to-work distance consistent.
• Plan a tack or skip sequence that spreads heat across the panel.
• Place a copper or aluminum backing bar where access allows.
• Run a test weld on scrap with the same joint and thickness.

Safety Before Welding Sheet Metal

MIG welding exposes you to arc radiation, hot metal, sparks, electrical hazards, noise, and fumes. Wear an appropriate welding helmet, safety glasses, welding gloves, flame-resistant clothing, and closed footwear. Use hearing protection when the task or surrounding work requires it.

Move combustible material away from the work or shield it from sparks and heat. Keep suitable fire-extinguishing equipment ready, and inspect the opposite side of walls, panels, floors, and other hidden areas that can ignite from conducted heat or falling sparks.

In a workplace, follow the employer’s hot-work, ventilation, respiratory-protection, and fire-watch requirements. OSHA requires adequate eye and face protection for arc welding and emphasizes controlling fumes through work practices, ventilation, and local exhaust. Outdoor work does not automatically guarantee safe fume control.

How to Tell When Your MIG Settings Are Correct

After a test weld cools, inspect the bead before moving to the final part. A usable thin-sheet setting should produce a controlled bead with consistent width, smooth tie-in at both edges, and enough penetration for the joint without a sagging back side or open hole.

• Too hot: The puddle grows quickly, the bead becomes excessively wide or concave, the edges undercut, or the back side sags.
• Too cold: The wire stubs, the bead is tall and rope-like, spatter increases, or the toes do not fuse into the sheet.
• Travel too slow: The bead is wide and the panel distorts or burns through.
• Travel too fast: The bead is narrow, irregular, or poorly tied in.
• Poor gas coverage: Pinholes, soot, or porosity appear; check flow, leaks, drafts, nozzle blockage, and stick-out.

Change only one variable at a time unless the machine uses a synergic control that links settings automatically. Retest after each change so you know which adjustment improved or harmed the weld.

Frequently Asked Questions

Why is my MIG welder burning through metal?

The joint is receiving more heat than the thin metal can absorb. Common causes include high voltage or wire feed, slow travel, a long or changing contact-tip-to-work distance, an open gap, repeated welding in one hot area, or sheet that has been ground too thin.

How do I weld sheet metal without burning through?

Start with the welder’s chart, use the smallest approved wire for the job, keep the joint tight, and test on matching scrap. Weld in short, separated sections with steady straight travel. Use a copper backing bar when possible and allow the panel to cool between nearby welds.

How do I fix burn-through in welding?

Let the metal cool, clean the damaged edges, and clamp copper behind the hole if access allows. Correct the settings first, then place very short tacks on sound metal around opposite sides of the opening. Build inward with cooling pauses instead of trying to fill the hole in one pass.

Can too much shielding gas cause porosity?

Yes. Excessive flow can create turbulence that pulls surrounding air into the shielding stream. Too little flow, drafts, leaks, or a blocked nozzle can also cause porosity. Use the wire and machine manufacturer’s recommended flow and verify gas delivery at the gun.

Does a copper backing plate stop MIG burn-through?

It helps by supporting the puddle and carrying heat away from the sheet, but it does not correct settings that are far too hot. The copper must sit clean and tight against the back of the joint to work well.

Is stitch welding better for thin sheet metal?

Stitch or skip welding is often better because it distributes heat instead of concentrating it along one continuous bead. Move between separated areas and allow cooling time. The weld length and spacing still need to match the joint’s design requirements.

What wire size is best for MIG welding thin sheet metal?

For thin mild steel, .023- or .025-inch solid wire is a common choice because it can run smoothly at lower current than larger wire. Confirm that the welder, liner, drive roll, contact tip, shielding gas, and filler-metal instructions support the selected size.

Should I lower voltage or wire speed first?

Use the machine’s chart and adjust in small steps. Wire feed speed strongly affects amperage on a typical constant-voltage MIG welder, while voltage affects arc length and bead shape. If the weld is too hot, reduce the appropriate setting without letting the arc become unstable, and retest on scrap.

Sources

1. Miller Electric: Successfully Welding Sheet Metal with MIG and TIG — supports fast straight travel, push technique, skip welding, tight fit-up, and backing bars.
2. Miller Electric: MIG Welding—Setting the Correct Parameters — supports the relationship among material thickness, wire size, wire feed, voltage, travel speed, and burn-through.
3. Miller Electric: What Type of Gas Is Best for MIG Welding in DIY Applications? — supports general short-circuit gas-flow guidance, turbulence, drafts, and contact-tip-to-work distance.
4. OSHA: Welding, Cutting, and Brazing—Hazards and Solutions — supports PPE, fume, radiation, burn, electrical, and other welding safety guidance.
5. OSHA 29 CFR 1910.252: General Requirements — supports fire prevention, eye protection, protective clothing, hot-work precautions, and container safety.

Conclusion

Preventing burn-through in MIG welding starts with controlled, stable heat. Use the machine’s settings chart, match the wire and gas to the material, keep the joint clean and tight, maintain a short consistent contact-tip-to-work distance, and move fast enough to keep the puddle small.

Practice on scrap with the same thickness and joint before welding the final part. If a hole forms, stop, cool the panel, correct the cause, and rebuild the opening with separated tacks over a backing bar. These steps produce cleaner welds with less distortion and far less frustration.