How To Fix Burn Through In Mig Welding – Professional Repairs

ByJim Boslice Metalworking & Fabrication

To fix burn-through, first clean the area thoroughly and place a copper or brass backing bar behind the hole to support the new molten metal. Use short, intermittent “tack” pulses to fill the gap from the edges inward, allowing the metal to cool between triggers to prevent further melting.

Once the hole is filled and slightly proud of the surface, use a flap disc on an angle grinder to smooth the weld flush with the base material for a seamless finish.

We have all been there: you are laying down a nice bead on a thin piece of sheet metal, and suddenly, the puddle disappears. You are left staring at a jagged, glowing hole where your joint used to be. It is a frustrating moment that can make any DIYer feel like they have ruined their entire project.

The good news is that learning how to fix burn through in mig welding is a fundamental skill that separates the beginners from the seasoned shop veterans. Metal is incredibly forgiving if you know how to manipulate heat and filler material to bridge the gaps you accidentally created.

In this guide, I will walk you through the exact steps to plug those holes, the tools that make the job easier, and the settings you need to adjust to stop it from happening again. Whether you are working on a classic car floor pan or a thin-walled square tube, you can make it look like the mistake never happened.

Understanding the Mechanics of Burn-Through

Burn-through occurs when the heat input from your welding torch exceeds the ability of the base metal to support the molten puddle. Instead of the metal fusing together, the base material reaches its liquidus point and simply falls away due to gravity or the force of the shielding gas.

This is most common when working with thin-gauge materials, such as 18-gauge or 22-gauge steel. At these thicknesses, there is very little mass to “soak up” the heat generated by the arc. If your travel speed is too slow or your voltage is too high, the metal disappears in the blink of an eye.

Before you attempt to fix the issue, you must understand that the metal around the hole is now thinner and more brittle. Your repair strategy needs to focus on heat management and adding filler metal without adding excessive localized temperature to the surrounding area.

Essential Tools for Repairing Welding Holes

You cannot effectively repair a burn-through with just your welder and a prayer. Having a few specific items in your arsenal will make the process significantly more successful and much less stressful for a beginner.

  • Copper Backing Bar: This is the secret weapon of pro welders. Steel will not bond to copper, but copper acts as a massive heat sink and a “floor” for your new weld puddle.
  • Angle Grinder with Flap Discs: You will need a 60 or 80-grit flap disc to grind the repair flush. Flap discs are more forgiving than hard grinding wheels on thin metal.
  • Wire Brush or Acetone: Cleanliness is vital. Any soot or oxidation around the hole will cause the repair weld to fail or pop.
  • Smaller Diameter Wire: If you are using 0.035 wire, consider switching to 0.023 or 0.030 wire for the repair. Thinner wire requires less heat to melt, which is exactly what you want here.

how to fix burn through in mig welding: The Step-by-Step Repair Process

When you realize a hole has formed, stop immediately. Do not try to “chase” the hole while the metal is still glowing red, as this usually just makes the hole larger. Let the workpiece cool down completely before starting the repair process.

Step 1: Prep the Area

The edges of a burn-through hole are often covered in silica deposits and soot. Use a wire brush or a clean file to remove all contamination from the perimeter of the hole. You want shiny, bare metal so the new weld can “bite” into the edges effectively.

Step 2: Position Your Backing

If you can reach the back of the workpiece, clamp a copper spoon or a flat piece of copper bar directly behind the hole. This provides a surface for the molten wire to land on. Since the weld won’t stick to the copper, you can simply pop the bar off once you are finished filling the gap.

Step 3: The “Tack and Cool” Method

Do not try to run a continuous bead to fill a hole. Instead, use a series of short pulses. Aim your wire at the thickest edge of the hole and pull the trigger for about half a second. Let the bright red glow fade to a dull orange before you hit it with the next pulse.

Step 4: Bridge the Gap

Continue adding these small “dots” of weld around the perimeter, moving inward. Each new tack should overlap the previous one slightly. By the time you reach the center, you will have a solid plug of metal filling the void. Ensure the filler is slightly higher than the surrounding surface.

Step 5: Grinding and Inspection

Once the area is cool to the touch, use your flap disc to grind the mound of weld metal down. Go slowly to avoid thinning the base metal further. Once flush, inspect for “pinholes” or porosity. If you see tiny black dots, you may need to add one more quick tack and regrind.

Why Does Burn-Through Happen in the First Place?

Fixing the mistake is one thing, but preventing it will save you hours of grinding in the long run. Most DIYers experience burn-through because of a few common set-up errors that are easily corrected with a bit of practice.

Excessive Voltage Settings

If your voltage is set too high for the thickness of the metal, the arc will be too “hot.” This causes the metal to liquefy faster than you can move the torch. Always refer to the door chart on your MIG welder as a starting point, then dial it back slightly if you are working on very thin material.

Slow Travel Speed

If you linger in one spot for too long, the heat builds up until the metal gives way. You must maintain a consistent travel speed. On thin metal, you generally want to move faster than you would on thick plate. Think of it like moving a candle under your hand—move fast and you’re fine; stop and you get burned.

Poor Fit-Up

A “gap” between two pieces of metal is an invitation for burn-through. If your pieces aren’t touching, the arc has nothing to stabilize on, and it will simply melt the edges away. Spend more time on your material preparation to ensure the joints are tight with no visible light passing through them.

Advanced Techniques: Using Heat Sinks and Chill Bars

When you are how to fix burn through in mig welding on critical components where aesthetics matter, you can use advanced heat management. A “chill bar” is a heavy piece of aluminum or copper clamped near the weld zone to draw heat away from the thin steel.

You can also use welding putty or “heat paste” around the area you are welding. These products act as a thermal barrier, preventing the heat from traveling too far into the surrounding sheet metal. This is particularly helpful when welding near painted surfaces or glass.

Another pro tip is to use the “stitch weld” technique. Instead of one long bead, weld one inch at the top, one inch at the bottom, and then move to a completely different part of the project. This allows the first sections to cool down before you return to finish the seam.

The Role of Shielding Gas and Wire Speed

Your machine settings play a massive role in how the puddle behaves. If your wire feed speed is too low while your voltage is high, the wire will melt back into the tip or “glob” off, often leading to a sudden burst of heat that blows a hole through the metal.

For thin DIY projects, a gas mixture of 75% Argon and 25% CO2 (C25) is the gold standard. Straight CO2 provides deeper penetration, which is the last thing you want when trying to avoid burn-through. The Argon content helps stabilize the arc and keeps the heat more localized on the surface.

If you find yourself constantly blowing holes, try increasing your wire speed slightly. This can actually cool the weld puddle because the cold wire acts as a heat sink as it enters the molten metal. It sounds counterintuitive, but more wire can sometimes prevent a blowout.

Safety Practices When Repairing Burn-Through

Repairing holes often involves a lot of re-striking the arc. This can lead to increased UV exposure and eye strain. Ensure your auto-darkening helmet is set to the correct shade (usually 9 or 10 for low-amp MIG) and that your sensors are clean.

Because you are often welding on smaller, thinner pieces, they get hot very quickly. Never attempt to hold a backing bar by hand; always use locking pliers or clamps. A piece of copper might not stick to the weld, but it will certainly blister your skin if you touch it after a repair.

Lastly, be mindful of the grinding sparks. When you are cleaning up a repair, you are often throwing a high volume of sparks in a small area. Ensure your workspace is clear of flammable materials, and always wear a face shield over your safety glasses during the grinding phase.

Frequently Asked Questions About how to fix burn through in mig welding

Can I fix burn-through without a copper backing bar?

Yes, you can, but it is much more difficult. Without backing, you have to rely entirely on surface tension and very fast trigger pulls. You will likely have to “bridge” the hole by starting on the edge and slowly building a ledge of metal until the hole is closed.

Does flux-core welding make burn-through worse?

Generally, yes. Flux-core arc welding (FCAW) typically runs hotter and produces more penetration than MIG with shielding gas. If you are using a flux-core machine on thin metal, you must be extremely careful with your settings and travel speed to avoid constant holes.

What should I do if the hole keeps getting bigger?

Stop welding immediately. This usually means the surrounding metal is too hot. Let it cool for five minutes. If the metal is extremely thin (like rusted body panels), you may need to cut out the thin section and weld in a “patch” of fresh, thicker metal rather than trying to fill a hole in “Swiss cheese” steel.

Is a repair weld as strong as the original metal?

If done correctly, yes. A plug weld used to fix a burn-through is essentially a solid piece of steel filler. As long as you have good fusion at the edges and you have ground it flush without thinning the surrounding area, the structural integrity will be maintained.

Summary of Key Takeaways

Mastering how to fix burn through in mig welding is an essential rite of passage for any garage fabricator. It teaches you the limits of your machine and the importance of heat management. Remember these core principles for your next project:

  • Cooling is key: Never rush a repair; let the metal lose its glow between pulses.
  • Back it up: Use copper or brass to provide a foundation for your repair puddle.
  • Cleanliness: A dirty hole will never weld shut properly; grind it to shiny metal first.
  • Settings: If you are burning through, increase your travel speed or lower your voltage.

Don’t get discouraged by a few holes in your work. Every professional welder has blown through metal more times than they can count. The difference is that the pros know exactly how to fill it, grind it, and move on. Grab some scrap sheet metal, intentionally blow a few holes in it, and practice these repair techniques today. You will find that with a little patience, those “mistakes” become invisible in no time. Happy welding!

Jim Boslice
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