Cold lap: the bead sits on top
Rounded, proud, with a visible line where it meets the parent metal — and it lifts off with a chisel because nothing fused. Cause: too little heat, or too much wire for the voltage. Increase voltage, slow the wire, or slow your travel.
Undercut: a groove along the edge
A trench cut into the parent metal at the toe of the weld, which is a stress raiser and a real defect. Cause: too much heat, traveling too fast, or the wrong gun angle. Reduce voltage, pause slightly at the edges, or steepen the angle.
Porosity: pinholes in the surface
Gas trapped as the pool froze. Causes, in order of likelihood: no gas or too much gas (turbulence pulls air in), a blocked nozzle, a draft across the work, or contamination — paint, oil, galvanizing. On flux-cored, porosity often means the polarity is wrong.
Excess spatter
Not a defect in itself but a symptom: too much voltage, too long a stick-out, or the wrong gas. On gasless flux-cored some spatter is normal; on MIG under an argon mix, heavy spatter means a setting is wrong.
The questions welders ask
What does a good bead look like?
Slightly domed rather than proud or sunken, with even ripples, fused smoothly into the parent metal at both toes without a visible line or a groove. On steel MIG the surface should be bright with a light dusting of silica islands, which brush off.
Can I fix a bad bead by welding over it?
Only if you remove it first. Welding over cold lap traps the unfused line inside the joint, where it will crack. Grind the bead out, clean back to bright metal, correct the setting and run it again — which is faster than it sounds and the only honest repair.
Last reviewed 2026-08-14
