Bubbly-looking welds are a sign of porosity — tiny gas pockets trapped in the weld metal as it solidifies. This is one of the most common welding defects for beginners and experienced welders alike, and the good news is that it's almost always preventable once you identify the root cause. Here's a practical guide to diagnosing and fixing the problem.
What Is Porosity and Why Does It Matter?
Porosity occurs when gas becomes trapped inside the weld pool before it can escape. The result is a weld bead that looks bubbly, pitted, or full of small holes — either on the surface or hidden underneath. Beyond looking bad, porosity weakens the weld significantly, reducing its load-bearing capacity and making it prone to cracking. For structural or safety-critical work, a porous weld is a failed weld.
The Most Common Causes of Bubbly Welds
1. Contaminated Base Metal
This is the number one culprit. Oil, grease, paint, rust, mill scale, moisture, and galvanising all release gases when heated, which get trapped in the weld pool. Always clean your base metal thoroughly before striking an arc.
- Use an angle grinder or wire brush to remove rust and mill scale
- Wipe surfaces with acetone or a dedicated degreaser to remove oil and grease
- Grind off any paint or galvanised coating in the weld zone (see safety note below)
- Dry any wet or damp metal before welding
2. Shielding Gas Problems (MIG/TIG Welding)
For MIG (GMAW) and TIG (GTAW) welding, shielding gas protects the weld pool from atmospheric oxygen and nitrogen. If this protection fails, you'll get porosity fast.
- Check your gas flow rate — typically 10–15 litres per minute for MIG, 6–12 L/min for TIG (check your machine manual)
- Inspect hoses and fittings for leaks
- Make sure your gas bottle isn't empty or nearly empty
- Avoid welding in windy conditions outdoors — wind blows shielding gas away. Use a windbreak or move inside
- Check that your contact tip and nozzle aren't clogged with spatter
3. Damp or Incorrect Electrodes (Stick Welding)
For stick (SMAW) welding, moisture in the electrode coating is a major cause of porosity. Store electrodes in a dry location — ideally in a sealed container or rod oven. Low-hydrogen electrodes (such as E7018) are especially moisture-sensitive and should be kept at the manufacturer's recommended temperature if possible.
4. Wrong Welding Parameters
Welding too fast, too slow, or with incorrect voltage and amperage can all contribute to porosity.
- Too fast: the weld pool doesn't have time to allow gas to escape
- Too slow: excessive heat can cause burn-through and draw in contaminants
- Voltage too low (MIG): poor arc stability and incomplete shielding
- Always refer to your machine's settings guide or the consumable manufacturer's data sheet for recommended parameters
5. Incorrect Technique
- Holding the torch or electrode at the wrong angle can disrupt gas coverage
- For MIG, a slight push angle (5–15°) generally gives better gas coverage than dragging
- Keeping too long an arc length in stick welding introduces atmospheric contamination
A Quick Diagnostic Table
| Symptom | Likely Cause | Fix |
|---|---|---|
| Porosity across whole bead | Contaminated metal | Clean and degrease thoroughly |
| Porosity at weld start | Cold/damp metal or electrode | Preheat metal; dry electrodes |
| Porosity in windy conditions | Shielding gas blown away | Use windbreak; increase flow rate |
| Random scattered pores | Gas leak in MIG setup | Check hoses, fittings, nozzle |
| Porosity with galvanised steel | Zinc vapour contamination | Grind coating off weld zone |
Safety: Welding Fumes Are Serious
Always weld in a well-ventilated area. Welding fumes — especially from coated, painted, or galvanised steel — are hazardous to your health. Zinc fumes from galvanised steel can cause metal fume fever. Use appropriate respiratory protection (at minimum a P2 respirator, or a powered air-purifying respirator for heavy work) and ensure good airflow. WorkSafe NZ has detailed guidance on welding fume hazards and exposure limits.
Improving Your Welds: A Practical Checklist
- ✅ Clean and degrease base metal before every weld
- ✅ Remove rust, paint, and coatings from the weld zone
- ✅ Store electrodes dry; use a rod oven for low-hydrogen rods
- ✅ Check shielding gas flow, hoses, and nozzle before starting
- ✅ Shield your work area from wind
- ✅ Set correct voltage, amperage, and wire speed per your consumable's data sheet
- ✅ Maintain correct torch/electrode angle and travel speed
- ✅ Preheat thicker or cold metal in winter conditions
When to Call a Professional
DIY welding is fine for non-structural projects like garden furniture or trailer repairs, but structural welding — including anything attached to a building, vehicle chassis, lifting equipment, or pressure vessels — should be carried out by a qualified tradesperson. In New Zealand, structural welding on buildings may require work by a certified welder under NZ Building Code requirements, and some work will require a building consent — always check with your local council first.
If you're unsure whether your welds are fit for purpose, or if you need certified weld inspection, a qualified welding inspector or structural engineer can assess your work. You can find experienced, local welders on tradedirectory.co.nz.
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