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MIG porosity, spatter and burn-through: causes and how to fix them

Porosity gets blamed on bad wire more often than it deserves. Almost always it's lost gas shielding or dirty metal. Spatter is voltage too low for the wire speed, or straight CO₂. Burn-through is too much heat for the gauge. WeldMaster's free Troubleshooting entries cover these three with causes ranked most to least common and fixes that start with the settings; 11 more defects come with Pro.

3 min read · Updated 2026-09-15

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What causes MIG welding porosity?

Porosity shows as pinholes or bubbles in the weld face, or as voids that appear when you grind. The surface can look pockmarked, like the top of a sponge. The app ranks the causes like this:

  • Lost gas shielding: flow too low, flow so high it turns turbulent, or wind blowing the gas away.
  • Dirty base metal: rust, mill scale, paint, oil or moisture.
  • Wrong or contaminated gas, or a leaking gas line.
  • Stick-out too long, or a nozzle clogged with spatter.

Fixes, settings first: bring the gas flow into range (MIG short-circuit 20–25 CFH, spray 30–35, TIG 15–20); block drafts, since anything over about 5 mph strips the shield; cut stick-out to about 3/8" and clean the nozzle; grind to bright metal before you strike an arc.

Why is my MIG welder splattering?

Excessive spatter means balls of metal thrown around the weld, stuck to the plate and the nozzle, and a rough bead that needs a lot of cleanup. The usual causes, in order:

  • Voltage too low for the wire speed: a cold, stubbing short-circuit.
  • Wire feed speed too high.
  • 100% CO₂ gas, which naturally spatters more than argon mixes.
  • Dirty metal or too much stick-out.

Fixes: raise the voltage one volt at a time until the harsh crackle turns into a steady sizzle; back the wire speed off slightly so it stops stubbing into the plate; switch from 100% CO₂ to C25 or a higher-argon mix; shorten stick-out to about 3/8" and clean the joint.

What causes burn-through in welding?

Burn-through is holes melted right through the work, or the puddle dropping out of the back. It is common on sheet metal and thin tube. The causes: amperage or heat input too high for the thickness, travel too slow, a root gap too wide, or wire or rod too big for thin material.

Fixes: drop to the low end of the range for the gauge; speed up and use a stitch or tack pattern so the sheet cools between beads; use .023 – .030 wire on sheet metal; close the gap and back the joint with a copper or aluminum bar to pull heat away and support the puddle.

How do you use Troubleshooting in WeldMaster?

  1. Open Troubleshooting under Reference.
  2. Tap the defect that matches your weld, for example Porosity.
  3. Compare your bead with the drawing: a good weld bead next to the defect, in cross-section.
  4. Read What it looks like to confirm you picked the right defect.
  5. Go through Likely causes (most → least common) from the top.
  6. Apply Fixes — settings first one at a time, running a test bead after each change.

Which weld defects does the chart cover?

Defects covered by WeldMaster's Troubleshooting list
DefectAccess
Porosity, Excessive Spatter, Burn-ThroughFree
Undercut, Lack of Fusion, Lack of PenetrationPro
Hot Cracking, Cold CrackingPro
Wire Birdnesting, Tungsten Contamination, Arc BlowPro
Warping / DistortionPro
Weld Bead Too Tall / Ropey, Weld Bead Too Flat / UndersizedPro

Every entry follows the same pattern of symptom, ranked causes and fixes, so the Pro entries read like the three free ones. The drawings are simple diagrams, not photos.

What are common mistakes when fixing a bad weld?

  • Changing three things at once because you're impatient. If the weld improves, you won't know which change did it. One thing, one bead, then decide. It is slower, but it avoids guessing which change worked.
  • Turning up the gas to fight wind. Too much flow turns turbulent and pulls air in; screen the draft instead.
  • Blaming the machine before cleaning the metal. Mill scale and oil cause porosity at any setting.
  • Grinding out a defect and welding over it with the same settings.

Questions and answers

Why does my MIG weld have holes in it?
Small bubbles are porosity, usually from lost gas coverage or dirty metal: check the flow, block drafts, shorten stick-out and clean to bright metal. Holes melted right through are burn-through: too much heat or too slow on thin metal.
Which troubleshooting entries are free?
Porosity, Excessive Spatter and Burn-Through. The other 11 defects, such as Undercut and Lack of Fusion, come with WeldMaster Pro.
Does C25 spatter less than 100% CO₂?
Yes, as a rule. The app lists 100% CO₂ as a spatter cause and suggests switching to C25 or a higher-argon mix for a softer arc.
Are there photos of real weld defects?
No. Each entry has a schematic cross-section drawing of a good bead next to the defect, plus the written symptom, causes and fixes.

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