Welding Porosity

Welding porosity is gas trapped in a weld. It can show up as small pinholes on the surface, larger rounded holes after grinding, or hidden pockets inside the weld. For beginners, porosity is one of the most common signs that something is wrong with cleaning, shielding, consumables, or technique.

Porosity matters because trapped gas can reduce weld quality and may point to poor preparation. A few pinholes on a practice coupon are feedback. Porosity on a structural, pressure, vehicle, trailer, lifting, or workplace weld needs qualified inspection and proper repair guidance.

Mark Dawson beginner note: do not only grind away porosity and keep welding. Find the source first. If the cause is still there, the next bead will usually have the same problem.

Quick Answer

Welding porosity is a defect caused by gas becoming trapped in the weld metal as it solidifies. Common causes include dirty base metal, oil, paint, rust, moisture, poor shielding gas coverage, wind, long arc length, contaminated filler, damp electrodes, wrong gas flow, or welding over coatings. To fix it, clean the metal, protect the weld from air, check gas flow and leaks, use dry consumables, shorten arc length, and retest on scrap.

Porosity Sign	Likely Cause	Beginner Fix
Pinholes on bead	Gas or contamination	Clean and check shielding
Holes after grinding	Internal gas pockets	Stop and diagnose
Porosity near start	Dirty start or weak gas	Clean and improve start
Porosity outdoors	Wind blowing shielding gas	Block wind or use right process
Porosity with stick rods	Damp rods or long arc	Use dry rods and shorter arc

What Porosity Looks Like

Surface porosity usually looks like small round holes in the weld bead. Some holes are tiny pinholes. Others are larger pits. Porosity may appear in a cluster, run along one side of the bead, or show only at the start or end.

Internal porosity is harder because it may not be visible from the outside. A weld can look acceptable on the surface and still have gas pockets inside. That is why critical welds may need testing beyond visual inspection.

If you are learning defect names, read Common Welding Defects and How to Check Welding Defects alongside this guide.

Why Porosity Matters

Porosity is not only an appearance problem. Gas holes can reduce the soundness of the weld and may indicate that the weld pool was exposed to air, moisture, oil, coating, or another contaminant. The size, amount, and location of porosity all matter.

On scrap practice, porosity is a lesson. On work that carries load, pressure, vibration, or safety risk, porosity can be a rejection issue. Acceptance depends on the job requirements, code, inspection method, and weld procedure.

Do not assume that a few surface pinholes are the whole problem. Grinding can reveal more holes below the surface, and internal porosity may require qualified testing to find.

What Causes Welding Porosity?

Porosity happens when gas enters the molten weld pool and gets trapped as the weld cools. The gas can come from contamination, poor shielding, moisture, coating, bad technique, or the wrong setup for the process.

• Oil, grease, paint, rust, mill scale, or moisture on the base metal.
• Wrong shielding gas, low flow, high flow turbulence, leaks, or drafts.
• Contaminated filler rod, dirty wire, or damp stick electrodes.
• Long arc length or poor torch, gun, or rod angle.
• Welding over galvanized, coated, plated, or unknown material.
• Poor starts and stops that leave the puddle unprotected.
• Gas nozzle, cup, or diffuser blocked by spatter or dirt.

MIG Welding Porosity

MIG porosity often starts with shielding gas. Check the cylinder, regulator, flow setting, hose, fittings, solenoid, gun liner area, diffuser, and nozzle. A loose fitting or blocked nozzle can let air reach the puddle even when the regulator looks normal.

Wind is another common cause. Gas MIG does not like drafts. A fan, open garage door, or outdoor breeze can push shielding gas away from the weld. If porosity appears only outdoors, wind is a strong suspect.

Also check the metal. MIG wire cannot solve oil, paint, wet steel, heavy rust, or dirty mill scale by itself. For MIG control basics, read MIG Welding Techniques for Beginners.

TIG Welding Porosity

TIG porosity can come from dirty base metal, contaminated filler rod, poor gas coverage, long arc length, a dirty tungsten, or pulling the torch away before post-flow protects the hot weld and tungsten.

Cleanliness is a bigger deal with TIG. Wipe filler rod, remove oil, brush aluminum oxide when welding aluminum, and regrind tungsten after dipping it. A contaminated tungsten can make the arc unstable and introduce dirt into the weld.

For TIG setup, review What Gas Do You Use for TIG Welding? and How to Grind Tungsten for TIG Welding.

Stick Welding Porosity

Stick welding porosity can come from damp electrodes, long arc length, dirty base metal, incorrect amperage, or poor rod angle. Low-hydrogen rods are especially sensitive to storage because moisture can affect weld quality.

Chip slag and inspect the cleaned bead. Some surface marks are slag-related, while real porosity is gas holes in the weld metal. Do not judge the bead fully until slag is removed.

For related rod care, see Low Hydrogen Electrode Storage and Stick Welding Basics for Beginners.

Flux Core Welding Porosity

Self-shielded flux core does not use external shielding gas, but it can still have porosity. Causes include wrong polarity, long stickout, dragging angle problems, moisture, dirty steel, wind beyond what the wire can handle, and poor technique.

Check the wire label and machine polarity. Many beginner flux core problems come from using the wrong polarity after switching from MIG. Also keep wire dry and protected from rust or shop moisture.

For process basics, read Flux Core Welding Tips for Beginners.

Porosity at the Start or End of the Weld

Porosity near the start can happen when the joint was dirty, gas flow had not stabilized, the arc started too long, or the initial puddle formed over contamination. Make sure the start area is cleaned, the gas is flowing, and the arc is controlled from the first second.

Porosity near the end may involve crater technique, pulling the torch or gun away too fast, or leaving the hot puddle unprotected. TIG welds especially need post-flow to protect the hot tungsten and weld area. MIG and flux core stops also need good crater control.

If porosity appears only at starts or stops, do not change every setting. Practice starts, restarts, and crater fill separately on scrap.

Gas Flow Mistakes That Cause Porosity

Low gas flow can leave the puddle exposed. High gas flow can create turbulence that pulls air into the shielding zone. Beginners often think more gas is always safer, but too much flow can make porosity worse.

Check the full gas path. A cylinder with gas in it does not guarantee coverage at the weld. Inspect the regulator, hose, fittings, torch or gun connection, nozzle, diffuser, cup, O-rings, and gas lens if used.

Workspace airflow matters too. Fans, open doors, compressed air, and outdoor wind can disturb gas coverage. If the same settings work indoors but fail near a doorway, the problem may be the environment.

How to Fix Porosity

1. Stop welding and mark where the porosity started.
2. Clean the base metal to bright, dry material where appropriate.
3. Check shielding gas flow, leaks, nozzle, cup, and drafts.
4. Use clean filler, wire, or dry electrodes.
5. Check polarity, stickout, arc length, and travel angle.
6. Remove the defective weld if the part requires repair.
7. Run a test bead on matching scrap before trying again.
8. Change one variable at a time so you know what fixed it.

On practice coupons, you can grind through a porous bead to see how deep the holes go. On real parts, repair depends on material, service requirement, defect size, and inspection rules. Do not guess on critical work.

How to Prevent Porosity

Prevention starts before the arc. Store consumables dry, clean the joint, remove coatings, confirm gas coverage, and test the setup on scrap. Keep nozzles and cups clean. Avoid welding through oil, paint, wet metal, or unknown residue.

During welding, keep arc length controlled and protect the puddle. For gas-shielded processes, avoid drafts. For flux and stick processes, use correct technique and remove slag between passes.

After welding, inspect before grinding everything smooth. If porosity appears, keep the failed coupon so you can compare it with the improved test bead.

Material and Surface Problems

Different materials bring different porosity risks. Mild steel may hide oil, paint, rust, mill scale, or moisture. Aluminum forms oxide and is sensitive to dirt on both the base metal and filler rod. Stainless steel needs clean surfaces and good shielding to avoid contamination and discoloration.

Coated or plated metals add extra risk. Galvanized, painted, oily, or unknown material can release hazardous fumes and contaminate the weld. Unknown coatings need proper identification, removal procedures, ventilation, and PPE before any welding starts.

Fit-up can also trap contamination. Oil or moisture inside a lap joint, tube seam, or tight corner may boil into the weld even if the top surface looks clean.

When Porosity Means Stop

Stop welding when porosity appears repeatedly after basic cleaning and setup checks. Repeating the bead without fixing the cause wastes material and may make repairs harder.

Stop immediately on critical welds. Trailer frames, vehicle parts, lifting points, pressure parts, structural members, and workplace welds need proper inspection and procedure-based repair. A beginner guess is not enough.

For practice, stop and write down what changed. Did porosity start after wind picked up, after a restart, after switching wire, after using an old rod, or after welding over a dirty section? The timing is often the clue.

How to Test Your Fix

After changing one likely cause, run a short bead on matching scrap. Use the same position, material thickness, and joint style if possible. If the porosity disappears, repeat the test before returning to the real part.

If the porosity improves but does not disappear, keep the successful change and check the next likely cause. For example, clean metal may reduce porosity, but a small gas leak may still leave occasional pinholes.

If nothing changes after several careful tests, step back and review the whole setup: process, polarity, consumable type, material coating, storage, gas, torch parts, and environment. A fresh look often catches the simple thing missed earlier.

Photograph the failed bead and the improved bead side by side. Surface holes, bead color, soot, and the location of the porosity can make the real cause easier to remember the next time it happens.

Keep notes on weather, gas flow, and material condition too, because porosity often appears when the shop environment changes.

That record helps separate a machine problem from a cleaning, storage, or airflow problem.

Small clues save a lot of wasted beads.

Porosity Troubleshooting Checklist

• Is the base metal clean, dry, and free from paint or oil?
• Is shielding gas correct for the process and material?
• Is the flow too low, too high, or affected by wind?
• Are the nozzle, cup, diffuser, or gas lens clean?
• Are filler rods, wire, or electrodes clean and dry?
• Is arc length or stickout too long?
• Is polarity correct for the wire or electrode?
• Did porosity start after a restart, crater, or torch movement change?

Common Beginner Mistakes

• Turning gas flow very high and creating turbulence.
• Welding near a fan or open door with gas MIG or TIG.
• Cleaning only the top surface while oil remains in the joint.
• Using rusty wire or damp stick electrodes.
• Grinding out porosity without fixing the cause.
• Blaming amperage before checking shielding and contamination.
• Welding coated or unknown material without proper safety steps.

Safety Notes

Porosity troubleshooting may involve grinding, rewelding, fumes, sparks, hot metal, electric shock, UV radiation, gas cylinders, and fire risk. Wear proper PPE, use ventilation, control sparks, and keep flammable material away.

Unknown coatings, sealed containers, galvanized material, pressure parts, vehicle frames, trailers, and lifting points require proper training and procedures before welding. Review OSHA welding hazards guidance, OSHA welding, cutting, and brazing requirements, and AWS safety resources.

FAQ

What is welding porosity?

Welding porosity is gas trapped inside the weld metal. It can appear as surface pinholes or hidden gas pockets inside the weld.

What causes porosity in welding?

Common causes include dirty metal, moisture, oil, paint, poor shielding gas, wind, contaminated filler, damp rods, long arc length, and wrong setup.

Can you weld over porosity?

Do not simply weld over porosity. Remove or repair the defective area as required, fix the cause, and retest. Critical welds need proper inspection and repair procedures.

Why do I get porosity with MIG welding?

MIG porosity often comes from weak gas coverage, wind, dirty metal, leaks, a dirty nozzle, wrong gas, long stickout, or contaminated wire.

Why do I get porosity with stick welding?

Stick porosity can come from damp rods, long arc length, dirty metal, wrong amperage, poor technique, or welding over contamination.

Final Advice

Welding porosity is a clue that gas entered the weld pool. Clean the metal, protect the puddle, check consumables, and fix the source before welding again. Once you learn to trace porosity back to its cause, troubleshooting becomes much faster.