You’re laying down a bead and it looks like Swiss cheese – small holes, pits, or even long tunnels running through the weld. The bead is ugly, and you’re not sure if it’s strong enough to hold.
That’s porosity. It’s trapped gas in the weld metal, and it’s one of the most common MIG welding defects. The good news is that most porosity has a straightforward cause, and you can fix it by checking a few things in the right order.
This guide gives you a step-by-step diagnostic workflow. Start with Step 1 (gas flow – it’s the most common cause) and work through. Most porosity is solved before you reach Step 4.
What Is MIG Welding Porosity?
Porosity is what happens when gas gets trapped in the weld metal as it solidifies. Instead of solid, sound metal, you get small holes, pitted surface, or long wormhole channels inside the bead.
The pattern of the porosity can tell you a lot about the cause.
| What It Looks Like | What It’s Called | Likely Cause |
|---|---|---|
| Tiny, evenly spaced pinholes across the bead surface | Uniform pinhole porosity | Contaminated base metal or wire |
| One or two long, tube-like cavities running inside the bead | Wormhole porosity | Insufficient or disrupted gas flow |
| Random scattered pits, larger than pinholes, irregular pattern | Scattered porosity | Draft/wind, technique issue, or intermittent gas problem |
| Crater at the end of the weld, pits concentrated there | Crater porosity | Stopped welding too abruptly – gas shield left too early |
Most MIG porosity falls into one of these patterns. If you see wormholes, your shielding gas is the first thing to check. If you see uniform pinholes, look at your base metal prep or wire condition.
Quick Answer – What Causes Porosity?
Porosity happens when the weld pool isn’t properly shielded from the air, or when contaminants in the weld zone create gas that gets trapped.
The most common causes of MIG porosity, in order of likelihood:
| Likelihood | Cause | Quick Fix |
|---|---|---|
| Most common | Insufficient or disrupted shielding gas flow | Check flowmeter, gas line, cylinder level |
| Very common | Contaminated base metal (rust, oil, paint, moisture) | Grind back to clean metal before welding |
| Common | Dirty or wrong wire | Check wire for rust, oil, or incorrect diameter |
| Less common | Nozzle spatter buildup or worn contact tip | Clean or replace nozzle and tip |
| Least common but real | Draft or wind blowing gas away | Move indoors, block airflow, or switch to flux-cored |
Always check gas first. It’s the easiest thing to rule out, and it’s the most common cause.
Step-by-Step Troubleshooting – Start Here
Follow these steps in order. Each step eliminates a set of possible causes. By the end of Step 3, you’ll have found the problem in most cases.
- Check gas flow – gas is the most common cause, and the fastest to check
- Check nozzle and contact tip – visual inspection takes 30 seconds
- Check wire condition – wire issues are easy to spot once you know what to look for
- Check base metal preparation – clean metal is essential for sound welds
- Check technique and environment – only after you’ve ruled out equipment
Step 1 – Check Your Gas Flow
This is where you start. Every time. Gas flow problems cause more porosity than everything else combined.
What to check:
- Flowmeter reading. Your flowmeter should show steady gas flow when you pull the trigger. For MIG welding mild steel with C25 gas (75% argon, 25% CO₂), a typical starting range is around 20 – 30 CFH (cubic feet per hour). Check your machine manual for its recommended range – it can vary by nozzle size, gun length, and welding position. Pure CO₂ may need slightly higher flow.
- Cylinder level. A nearly empty cylinder can cause intermittent flow. If the gauge reading is low, swap tanks.
- Gas line condition. Look for kinks, cuts, or loose fittings along the gas line from the regulator to the machine.
- Gas mixture. Verify you’re using the right gas for your material. C25 is standard for mild steel. Pure CO₂ works but produces more spatter and may need different settings.
What not to do:
- Don’t crank up the flow rate to compensate for a leak – fix the leak first
- Don’t disable the gas solenoid as a test
| Symptom | Likely Cause | Check / Fix |
|---|---|---|
| Flowmeter reads zero | Cylinder empty or valve closed | Open valve; swap cylinder if low |
| Flowmeter needle fluctuates | Loose fitting or kinked hose | Inspect line; tighten or replace |
| Adequate flow but porosity | Wrong gas mix or flow too high (turbulence) | Check gas label; reduce flow to manual spec |
| Porosity only on a long weld | Possible freeze-up on CO₂ regulator | Check for regulator icing |
When to check the manual: Your machine manual may specify a different flow range for your nozzle size, gun length, or welding position. Always confirm.
Step 2 – Check Your Nozzle and Contact Tip
A blocked or worn nozzle restricts gas flow even when the flowmeter reads fine. The gas can’t reach the weld pool.
What to check:
- Spatter buildup inside the nozzle. Spatter can clog the opening and create turbulent gas flow. Clean the nozzle with pliers or a nozzle cleaning tool.
- Contact tip bore. If the tip is worn oval or has spatter stuck in the bore, the wire won’t feed straight. This can affect gas coverage.
- Gas diffuser. The diffuser sits between the nozzle and the gun. If the holes are clogged, gas distribution is uneven.
- Nozzle size. A nozzle that is too small for the gas flow can create turbulence.
- ☐ Nozzle is free of visible spatter buildup
- ☐ Contact tip bore is round, not oval or plugged
- ☐ Gas diffuser holes are clean and open
- ☐ Nozzle is seated correctly and tight
- ☐ Nozzle interior is clean and smooth
Spraying anti-spatter compound on the nozzle and tip before each weld session helps prevent buildup. But check the compound directions – some need to be applied to a cool nozzle.
Step 3 – Check Your Wire
Wire issues cause porosity by introducing contamination into the weld pool or by feeding inconsistently.
What to check:
- Wire surface. Rust, oxidation, or oil on the wire surface contaminates the weld. If the wire looks discolored or feels greasy, replace the spool.
- Wire diameter. The contact tip, drive rolls, and wire must all match. A 0.030-inch wire in a 0.035-inch tip creates an inconsistent arc and poor gas coverage.
- Spool tension. If the spool drags or binds, the wire feeds in jerks, which can pull air into the gas shield.
- Wire storage. Wire absorbs moisture from the air. Store spools in a dry area.
| Wire Issue | What to Look For | Fix |
|---|---|---|
| Rust or oxidation | Brownish discoloration on wire surface | Replace the spool |
| Oil or grease | Shiny, slick film on wire | Replace the spool; check drive rolls for contamination |
| Wrong diameter | Wire doesn’t fit snugly in contact tip | Match wire, tip, and drive rolls to same diameter |
| Wire kinked or birdnested | Tangled wire at the drive rolls | Cut wire free, re-feed, check drive roll tension |
Gas type note: If you’re using pure CO₂, the arc is hotter and wire contamination can cause more noticeable porosity than with C25. Keep the wire clean.
Step 4 – Check Your Base Metal Prep
The base metal surface can introduce contaminants directly into the weld pool. Even contaminants you can’t see – like thin oil film – can cause porosity.
What to check:
- Visible contamination. Rust, paint, oil, grease, or mill scale on the weld surface. Grind all contamination away from the weld joint area – a good rule of thumb is to clean at least an inch on each side of the joint, though the exact distance depends on material thickness and joint type.
- Moisture. Any moisture on the metal will turn to steam in the arc and create gas pockets. The metal should be dry before welding.
- Aluminum oxide. Aluminum forms an oxide layer that melts at a much higher temperature than the base metal. It must be removed with a stainless steel brush before welding.
- Galvanized coating. Zinc coating on galvanized steel creates toxic fumes and can cause porosity. If you must weld galvanized steel, use proper ventilation or respirator, and grind the coating away from the weld zone. Follow OSHA guidelines for welding galvanized metal.
- ☐ Weld area is ground clean – no rust, paint, or mill scale (at least an inch each side of joint as a general guide)
- ☐ Metal surface is dry
- ☐ If aluminum: oxide layer removed with dedicated stainless steel brush
- ☐ If galvanized: coating ground away; ventilation or respirator in place
- ☐ No oil or grease near the weld joint
Draft / wind check: If you’re working near a fan or open door, even a light breeze can blow away shielding gas. Move to a sheltered spot or close the door.
Step 5 – Check Your Technique and Environment
If gas, nozzle, wire, and base metal all check out, the problem may be how you’re welding – or where.
What to check:
- Gun angle. If you’re holding the gun at too steep an angle, the nozzle can aspirate air into the gas stream. A generally recommended starting point is to keep the gun angle fairly upright – most short circuit MIG work uses a slight push or drag angle, often in the 10 – 15 degree range off vertical. Check your machine manual or a welding handbook for the right angle for your joint type.
- Travel speed. Moving too fast can outrun the gas shield. The gas shield forms a cone around the arc, and if you move faster than it covers, the trailing edge of the weld pool is exposed to air.
- Contact tip to work distance (stick-out). If the tip-to-work distance is too long, the gas stream spreads out and coverage drops. A general reference for most MIG applications is to keep stick-out around 1/2 to 3/4 inch – but again, check your manual for the recommended range for your wire diameter and settings.
- Drafts and wind. MIG shielding gas is easily disrupted by air movement. Even a light breeze from a shop fan or open door can cause porosity. This is especially true for CO₂ mixes, which are heavier than air and can flow away from the weld. If you must weld in a drafty area, use a wind screen or switch to flux-cored wire.
- Indoor vs outdoor MIG. MIG is designed to work with a shielding gas that can be blown away by air movement. If you’re welding outdoors on a breezy day, flux-cored wire (with or without gas) handles wind much better.
| Problem | Symptom | Fix |
|---|---|---|
| Steep gun angle | Porosity on one side of the bead | Adjust to a more upright angle; check manual for your joint type |
| Travel speed too fast | Narrow bead with porosity at edges | Slow down; watch the puddle fill |
| Tip-to-work distance too long | Weld sounds rough, lots of spatter, porosity | Reduce stick-out; check manual for recommended range |
| Draft or wind | Scattered porosity, sometimes intermittent | Block airflow; move indoors; switch to flux-cored |
Printable Workshop Checklist
Tape this to your toolbox or welding cart. Run through it whenever you see porosity.
Before you weld:
- ☐ Metal is clean – ground to bright metal away from joint
- ☐ Metal is dry
- ☐ Gas cylinder is full and valve open
- ☐ Flowmeter reads within recommended range (check manual)
- ☐ No kinks or leaks in gas line
If you see porosity during welding:
- ☐ Stop and check gas flow first
- ☐ Inspect nozzle for spatter buildup – clean if needed
- ☐ Check contact tip condition – replace if worn
- ☐ Check wire for rust, oil, or binding
- ☐ Reduce travel speed if needed
- ☐ Check for drafts or wind
- ☐ Grind out the porous section before welding over it
Common Misconceptions About Porosity
A few myths lead welders to chase the wrong fix. Here are the ones that waste the most time.
| Myth | Fact |
|---|---|
| “More gas flow means better shielding.” | Too much gas creates turbulence that pulls surrounding air into the weld pool. Stick to the range in your manual. |
| “Porosity is always a gas problem.” | Gas is the most common cause, but wire condition, base metal prep, and technique matter too. Work through all five steps. |
| “If the metal looks clean, it’s clean.” | Thin oil or grease film can be invisible and still cause porosity. Use acetone or a dedicated degreaser if the metal has been handled or machined. |
| “You need expensive gas for good welds.” | C25 (75% argon, 25% CO₂) is standard for mild steel and affordable. Pure CO₂ is cheaper and works, but produces more spatter and may need volume adjustments. |
When to Worry About Porosity (Structural vs Cosmetic)
Not every porous weld needs to be ground out and redone. But telling the difference between cosmetic and structural porosity requires caution.
If the weld is non-critical – a garden gate, a decorative piece, a repair that isn’t load-bearing – a few scattered surface pinholes on an otherwise sound weld may be acceptable, provided the weld has good penetration otherwise.
If the weld is load-bearing or safety-critical – a trailer, a vehicle, machinery, a handrail, a lifting point, or anything that could cause injury if it fails – any visible porosity is a reason to be cautious. Grind out the porous section and re-weld. When in doubt, a certified welder or engineer should evaluate the weld.
Rule of thumb: If you’re unsure whether the weld is strong enough, grind it out and do it over. The time it takes to re-weld is nothing compared to the cost of a failure.
⚠️ ⚠️ Important: This guide covers common troubleshooting for informational purposes. It does not replace professional welding inspection or engineering judgment. If the weld is load-bearing, safety-critical, or part of a structure that could cause injury if it fails, consult a certified welder or a qualified engineer. Do not rely on general guidance alone for structural welds.
FAQ
Can you weld over porosity?
No. Welding over a porous weld traps the existing voids deeper. Grind the porous weld out completely before re-welding. The goal is sound metal, not covering up the problem.
Does wind really cause porosity in MIG?
Yes. Even a light breeze can disrupt the shielding gas. A shop fan, an open door, or an outdoor breeze are all enough. MIG shielding gas is heavier than air and flows away from the weld in moving air. If you can’t block the wind, switch to flux-cored wire.
Can a bad ground cause porosity?
Indirectly. A poor ground connection affects arc stability. An unstable arc can contribute to porosity, but it’s not a direct cause. If you’ve checked everything else and still see porosity, verify your ground clamp is firmly attached to clean metal.
Conclusion
Porosity in MIG welding is frustrating but almost always fixable. The key is checking causes in the right order – start with gas flow, then work through nozzle, wire, base metal, and technique. Most of the time, you’ll find the problem by the end of Step 3.
Print the checklist from this guide and keep it in your shop. Run through it every time you see porosity, and you’ll spend less time troubleshooting and more time welding.
⚠️ Related guides: Check out the guide on MIG welding burnback – another common troubleshooting topic for MIG welders. More MIG troubleshooting articles are coming, including spatter control techniques.
