Most MIG welding defects come from five root causes: wrong settings, bad shielding gas coverage, contaminated base metal, worn consumables, or incorrect technique. If your welds look bad, the fix is almost always one of those five things. You don’t need to replace the machine. You need to diagnose the actual problem.
This guide covers each common MIG welding defect with its causes and specific fixes. Work through them systematically. Fix one variable at a time and run a test bead between each change. Changing multiple settings at once makes it impossible to know what actually solved the problem.
Porosity
Porosity shows up as small holes (pinholes), clusters of pits (surface porosity), or elongated cavities (worm tracks) in the weld bead. Gas gets trapped in the solidifying weld pool and can’t escape before the metal freezes.
Causes and Fixes
Contaminated base metal. Rust, mill scale, paint, oil, grease, marker ink, or moisture on the joint surface releases gas when the arc hits it. Fix: Clean every joint to bright, bare metal using a grinder, wire wheel, or flap disc. Wipe off any oils with acetone. For rusty or galvanized material, cleaning is non-negotiable.
Insufficient shielding gas coverage. Wind, fans, drafts from open doors, or even walking past the weld area can blow shielding gas away from the puddle. Fix: Block wind with welding screens or tarps. Increase gas flow rate by 5 CFH. If welding outdoors, use flux-cored wire (FCAW) instead.
Gas flow rate wrong. Too little gas means inadequate coverage. Too much gas (above 45-50 CFH) creates turbulence at the nozzle that pulls ambient air into the gas stream. Both cause porosity. Fix: Set flow to 25-35 CFH for most indoor work. Use the minimum effective flow rate.
Nozzle clogged with spatter. Spatter buildup inside the nozzle restricts and redirects gas flow. Fix: Clean the nozzle between passes with a welding pliers or nozzle reamer. Apply anti-spatter dip or spray.
Gas hose leak. A pinhole in the gas hose or a loose fitting lets air into the gas line. Fix: Pressurize the system with the gun trigger pulled and listen/feel for leaks at every connection. Replace damaged hoses.
Wet or contaminated wire. Wire left exposed to humidity absorbs moisture. Corroded wire has surface oxides that release gas. Fix: Store wire in sealed bags when not in use. Replace wire that looks discolored or feels rough.
Excessive Spatter
Some spatter is normal in short circuit MIG transfer. Excessive spatter that coats the workpiece, clogs the nozzle, and makes a mess is a settings or technique problem.
Causes and Fixes
Voltage too low for wire speed. This is the number one cause. The wire jams into the pool faster than the arc can melt it, causing violent short circuits and heavy spatter. Fix: Increase voltage by 1-2V increments or decrease wire speed. The arc should sound like a steady sizzle, not a machine gun.
Voltage too high. Excessive voltage creates a long, harsh arc that blows metal out of the puddle. Fix: Reduce voltage until the arc tightens and the sound smooths out.
Wrong polarity. MIG welding uses DCEP (electrode positive). Running DCEN or AC creates extreme spatter and poor penetration. Fix: Verify the gun lead is on the positive terminal.
Too much stick-out. Contact tip to work distance (CTWD) beyond 3/4 inch increases resistance, reduces current, and creates an unstable arc. Fix: Maintain 3/8 to 1/2 inch stick-out for short circuit transfer.
100% CO2 gas. Straight CO2 produces more spatter than 75/25 argon/CO2 by nature. The arc is more forceful and the transfer is more violent. Fix: Switch to 75/25 if spatter is unacceptable. If you need to stay on CO2 for cost or penetration reasons, accept some additional spatter and use anti-spatter compound.
Dirty base metal. Oil, rust, and coatings cause the arc to behave erratically, spraying molten metal everywhere. Fix: Clean the joint.
Undercut
Undercut is a groove or channel melted into the base metal alongside the weld bead that isn’t filled with weld metal. It weakens the joint by reducing the base metal cross-section and creates a stress riser that can initiate cracks.
Causes and Fixes
Travel speed too fast. The arc melts the base metal edge, but you move past before the filler metal fills the groove. Fix: Slow down. Let the puddle wet out to the toes of the weld before advancing.
Voltage too high. Excessive arc length melts a wider area than the filler metal can fill. Fix: Reduce voltage 1-2V.
Incorrect gun angle. On fillet welds, favoring one leg over the other directs more heat to one side, causing undercut on the opposite side. Fix: Split the angle evenly between both plates (45 degrees for T-joints). For butt joints, keep the gun perpendicular to the joint with a 10-15 degree push angle.
Excessive weave width. Weaving too wide spreads the heat over an area the puddle can’t fill. Fix: Tighten the weave or switch to stringer beads.
Wrong wire speed. Not enough filler metal for the heat input leaves unfilled grooves. Fix: Increase wire speed to provide more fill metal.
Burn-Through
Burn-through means the arc melted completely through the base material, leaving a hole. This is most common on thin sheet metal (16 gauge and thinner) and root passes on open joints.
Causes and Fixes
Heat input too high. Voltage, wire speed, or both are set too aggressively for the material thickness. Fix: Reduce voltage and wire speed. On thin material, start at the lowest settings that maintain an arc and work up.
Travel speed too slow. Dwelling in one spot concentrates heat and burns through. Fix: Move faster. On thin material, keep a steady pace. If you can’t move fast enough without losing the bead, reduce your wire speed.
Poor fit-up with gaps. Gaps in the joint give the arc a direct path through the material. Fix: Close gaps with better clamping or tacking. If gaps are unavoidable, skip the gap sections, let them cool, then fill from the edges inward.
Wrong wire diameter. Running 0.035" wire on 22-gauge sheet metal puts too much heat into too little material. Fix: Use 0.023" or 0.030" wire for material under 16 gauge.
No backing. Some thin-material joints need a copper or ceramic backing bar to support the weld pool. Fix: Use a backing strip or weld in shorter segments, allowing cool-down between each.
Lack of Fusion (Cold Lap)
Lack of fusion means the weld metal didn’t properly bond to the base metal or to a previous weld pass. The bead sits on top of the material or rolls over a previous pass without actually melting into it. This is a serious structural defect because the joint has zero strength at the unfused area.
Causes and Fixes
Heat input too low. The arc doesn’t generate enough energy to melt the base metal surface. Fix: Increase voltage first, then wire speed. You need both adequate voltage for penetration and enough wire speed to maintain the arc.
Travel speed too fast. Moving quickly prevents the arc from heating the base metal enough to achieve fusion. Fix: Slow down until you see the puddle actually flowing into the base metal at the leading edge.
Wrong gun angle. Pointing the gun too far ahead of the puddle or at the wrong plate in a fillet weld directs heat away from where fusion is needed. Fix: Direct the arc at the leading edge of the puddle where it meets the base metal.
Thick oxide or mill scale. Heavy scale acts as a barrier between the weld metal and base metal. Fix: Grind to bright metal at the joint.
Cold base metal in winter. Welding on steel below 32 degrees F (0 degrees C) pulls heat away so fast that fusion can’t occur at normal settings. Fix: Preheat the joint to at least 50-70 degrees F (10-20 degrees C) with a torch. On thicker material (1/2 inch+), preheat to 200-300 degrees F (93-150 degrees C) per code requirements.
Bird-Nesting (Wire Tangle at Drive Rolls)
Bird-nesting is when the wire tangles into a ball at the drive rolls inside the feeder. The wire can’t advance through the liner but the drive motor keeps pushing, so the wire bunches up. Production stops until you cut out the tangle and re-thread the wire.
Causes and Fixes
Clogged or kinked liner. Metal shavings accumulate inside the liner over time and create friction. A bent gun cable kinks the liner. Fix: Replace the liner every few months with heavy use. Keep the gun cable in gentle curves, not sharp bends. Blow out the liner with compressed air during wire spool changes.
Wrong liner size. Using a 0.035" liner with 0.030" wire allows the wire to wander and jam. Using a 0.030" liner with 0.035" wire creates excessive friction. Fix: Match the liner to the wire diameter.
Worn contact tip. An oblong or oversized contact tip hole causes the wire to arc inside the tip, welding itself in place. The drive rolls keep pushing against a stuck wire, and the wire tangles. Fix: Replace contact tips regularly. They’re consumable items. Keep spares on hand.
Drive roll tension too tight. Excessive tension deforms the wire, creating flat spots that jam in the liner and tip. Fix: Set drive roll tension just tight enough to feed wire reliably. If you can’t stop the wire by lightly pinching it with gloved fingers, the tension is too high.
Drive roll tension too loose. The rolls slip on the wire, causing intermittent feed and eventually a tangle. Fix: Increase tension until feed is consistent. But don’t go past the point where the wire deforms.
Wrong drive roll type. Use V-groove rolls for solid steel wire, U-groove rolls for aluminum, and knurled rolls for flux-cored wire. Using knurled rolls on solid wire chews up the wire surface and creates metal shavings that clog the liner. Fix: Match drive roll type to wire type.
Wire Stubbing
The wire hits the puddle and pushes the gun back instead of melting smoothly. You get a rhythmic “tapping” or “bumping” sensation in the gun, and the weld bead looks inconsistent.
Causes and Fixes
Voltage too low. Not enough arc energy to melt the wire as fast as it feeds. Fix: Increase voltage 1-2V.
Wire speed too high. The wire feeds faster than the arc can melt it. Fix: Reduce wire speed 25-50 IPM at a time.
Contact tip recessed too far. If the contact tip is set too far inside the nozzle, the wire stick-out is too long by the time it reaches the work. Fix: Adjust the contact tip position per the manufacturer’s recommendation, usually flush with or slightly recessed from the nozzle end.
Whiskers (Wire Sticking Through the Weld)
Short pieces of unmelted wire poking through the finished weld bead. These are sharp, dangerous, and indicate the wire didn’t fully melt at the end of a weld.
Causes and Fixes
Wire feeds after trigger release. Some wire feeders have a slight delay on shut-off. Fix: Release the trigger a fraction of a second before you reach the end of the bead. This takes practice to time correctly.
Burnback setting too short. Modern machines have an adjustable burnback time that keeps the arc lit briefly after the wire feeder stops. This melts the wire tip cleanly. Fix: Increase the burnback time in the machine settings. Start with the manufacturer’s default and adjust from there.
Pulling off the joint too fast. If you whip the gun away at the end of a bead, the wire can stick out before it melts. Fix: Hold the gun in position for a half-second after releasing the trigger to let the burnback do its job.
Systematic Troubleshooting Approach
When a weld doesn’t look right, resist the urge to randomly twist knobs. Use this sequence:
- Check consumables first. Contact tip, nozzle, liner, drive rolls. These are cheap and wear out constantly. Replace anything questionable.
- Verify gas. Check flow rate at the nozzle with a flow meter (not just the regulator gauge). Look for leaks. Confirm the right gas type.
- Clean the joint. Grind to bright metal. Wipe with acetone. Eliminate contamination as a variable.
- Check settings. Compare your voltage and wire speed to recommended charts for the material thickness. Start at the low end and adjust up.
- Evaluate technique. Travel speed, gun angle, stick-out, and work angle all affect weld quality. Run a bead on scrap with deliberate focus on each variable.
- Run a test bead. Change one variable at a time. Run a test bead. Evaluate. Repeat.
Most problems trace back to steps 1-3. Worn consumables and contamination cause more shop grief than settings or technique combined. Build a habit of replacing contact tips and cleaning nozzles before they cause problems, and half your troubleshooting work disappears.