Why MIG Weld Porosity Persists After Cleaning (And Fix It)

If you're trying to figure out how to fix a MIG weld with porosity after cleaning, you're probably staring at a bead that looks like a sponge. That's frustrating. You did the prep work.

You wiped the steel down. And still, the tiny pinholes show up. When porosity survives a proper cleaning, the problem is rarely the dirt.

Manufacturer specifications and welding standards from the American Welding Society point elsewhere. The usual suspects are shielding gas delivery, contaminated wire, and a worn contact tip. As of 2026, those three still cause most porosity complaints.

Before you re-clean the metal again, work through the gas and wire side of the equation first. That's where the real fix lives.

The Problem: You Cleaned the Metal and It Still Looks Like a Swiss Cheese

Porosity is the name for those small gas pockets that get trapped in the weld metal. It's one of the most common defects in MIG welding. And it's usually blamed on a dirty workpiece.

That's fair, because oil, rust, mill scale, and moisture all cause porosity. But if you've already cleaned the joint and the problem won't go away, you need to look at the process itself.

Think of the weld pool as a liquid. It can only hold so much dissolved gas before bubbles start forming. Those bubbles rise, and normally they escape before the metal freezes.

When they get stuck, you end up with a spongy bead. Anything that adds gas to the pool, or blocks gas from escaping, will cause porosity. The base metal is only one possible source.

So what's left after the steel is shiny and solvent-wiped? The shielding gas, the filler wire, and the tooling that delivers both. Those are the three areas that create porosity when everything else looks right.

If you're new to the process, reviewing the basics of MIG welding can help you understand why gas coverage matters so much.

The Quick Answer: What to Check Right Now

Check the shielding gas first. Then swap the contact tip. Then look at the wire.

Do this in order. Most porosity disappears at step one or two.

That order matters because gas problems can ruin every weld you make. A bad tip usually creates irregular pinholes. Dirty wire leaves small, evenly spaced holes.

Working through this sequence eliminates the most common causes before you touch your voltage or wire speed.

Here's the shortest version of the routine:

• Verify gas flow at the nozzle, not just at the regulator. You want 20 to 30 CFH indoors.
• Spray soapy water on all gas connections. Bubbles mean a leak.
• Replace the contact tip if the hole looks enlarged or the wire wobbles.
• Cut off a few inches of wire from the spool and discard it. That removes any surface rust or oil.
• Slow your travel speed slightly and keep a 10 to 15 degree push angle.

Use this table as a quick decision guide:

If you're welding outdoors, raise the flow to 35 to 45 CFH and use a wind screen. If the bead still has holes after that, check the tip next.

Why Clean Base Metal Still Causes Porosity: The Core Explanation

Here's the part that confuses a lot of welders. A clean base metal doesn't guarantee a clean weld. In Gas Metal Arc Welding, the weld pool needs a blanket of shielding gas to keep oxygen and nitrogen out.

That blanket comes from the nozzle, and it has to stay stable until the puddle freezes.

Think of gas coverage as an invisible umbrella. If the umbrella is the right size and stays in place, the puddle stays safe. If it tears, shifts, or gets pushed away, air makes its way in and bubbles form.

Those bubbles are porosity. So the condition of the metal is only half the story. The other half is what's happening above the joint.

Solid MIG wire relies entirely on external shielding gas. Flux-cored wire can generate its own shield, which is why it behaves differently in drafty conditions. Understanding the difference between the two shielding methods can save you from chasing the wrong problem.

If you're running solid wire and the gas blanket fails, no amount of base metal cleaning will fix the weld.

There's also a way cleaning itself causes issues. Aggressive grinding can embed abrasive grit into the surface, especially on thin sheet metal. That grit stays behind even after you wipe with solvent.

So clean the joint, but don't attack it with a grinder any harder than you have to.

First Diagnosis Step: Shielding Gas Supply Check

Shielding gas is the first suspect after cleaning because it's invisible and easy to miss. The gas regulator might read 25 CFH, but that number means nothing if the gas isn't actually reaching the arc.

Here's how to check the system properly:

• Turn on the cylinder and let the regulator settle.
• Set your flow meter anywhere between 20 and 30 CFH for indoor work.
• Put your hand near the nozzle and feel for gas while you pull the trigger.
• Listen for hissing at the connections. Hissing means a leak.
• Mix a little soapy water and brush it on every fitting. Bubbles reveal the leak.

If the flow at the regulator is strong but weak at the nozzle, the hose, liner, or torch assembly has a restriction. That's rare, but it happens. Replace the hose or check for a kink.

Wind and drafts are a quieter problem. A slight breeze can push the gas shield away from the puddle, and you won't notice it because the air feels still indoors. Ceiling fans, HVAC vents, and open bay doors all count.

If you suspect airflow, raise the flow rate or set up a simple welding screen.

Gas flow that's too high causes its own issue. Cranking the regulator to 50 CFH creates turbulence around the nozzle, and that turbulence pulls in air. More gas isn't always better.

Stick to the manufacturer's range and adjust from there.

If you're welding stainless steel, the gas mix changes completely. A standard C25 blend works for mild steel, but stainless steel has its own gas requirements. Using the wrong mix can produce a charred, porous bead that looks like you skipped cleaning entirely.

And don't forget the basics of cylinder safety. Keep bottles chained or strapped upright, and never weld in a tight space without ventilation. OSHA's welding safety guidelines cover cylinder handling and fume control, so it's worth a quick read.

The Gas Nozzle and Contact Tip: Inspect or Replace

A worn contact tip is the cheapest fix in welding. It's also the one most people ignore. The contact tip carries welding current to the wire.

Every time the arc starts, the tip wears a little. Eventually the hole gets oversized, the wire starts to wobble, and the arc gets unstable. An unstable arc means the shielding gas isn't protecting the puddle properly.

Porosity follows.

Here's how to inspect it. Remove the nozzle and look at the tip hole. If it's oval, enlarged, or stretched, replace it.

If there's heavy spatter welded to the outside, clean it off with a file or wire brush. A spatter-covered tip blocks gas flow and disrupts the shielding pattern.

The contact tip must match your wire diameter exactly. A 0.035 inch tip with 0.030 inch wire will give you an erratic arc every time.

The nozzle deserves attention too. Spatter builds up on the inside of the nozzle and constricts gas flow. A clogged nozzle can turn a clean bead into a leaky, porous mess.

Keep the inside of the nozzle clear, and replace it if it's dented or warped.

Stick-out length is part of this equation. That's the distance from the contact tip to the arc, and it should stay around 3/8 to 1/2 inch. If you hold the gun too far away, the gas blanket gets thin before it reaches the puddle.

If you get too close, the tip can melt into the weld. Finding that sweet spot fixes a surprising number of porosity problems.

Once you've checked gas and swapped the tip, move on to the wire itself. Rusty wire, oily wire, or wire that's been sitting in a humid garage can contaminate the weld pool all by itself. For a quick test, clip off six inches of wire and run it through a clean rag.

If the rag comes away brown or greasy, that's your answer. Replace the spool and keep the new one dry. To see how all these components fit together, it helps to know the parts of a welding torch.