A newer MIG welder at a fab shop noticed a recurring defect: small pinholes appearing at the termination of his welds. He posted photos hoping the more experienced crowd could diagnose it — and they did, in detail. The thread turned into a small clinic on weld terminations, shielding gas behavior, and crater fill technique.

The diagnosis from veteran welders converged on a few overlapping causes:

• Crater porosity — When you release the trigger, the puddle shrinks rapidly. Gases dissolved in the molten metal (CO₂, hydrogen, nitrogen) get trapped as the puddle freezes too fast to let them escape. The result: pinholes, often arranged in a tight cluster at the end of the bead.
• Loss of shielding gas at the stop — Pulling the gun away too quickly removes the gas cup before the puddle has solidified. Atmospheric nitrogen and oxygen rush in and contaminate the still-liquid metal. Several commenters emphasized holding the gun in position for 1–2 seconds after releasing the trigger to let post-flow do its job.
• No crater fill technique — Experienced welders described "back-stepping" or whipping back into the puddle just before stopping. This builds up the crater so it doesn't form a thin, gas-trapping pocket as it cools.
• Contamination — Mill scale, oil, galvanizing residue, or moisture on the base metal can outgas during the weld. The end of the bead is where you most often see it because that's where heat builds up and the puddle stays liquid longest.

One particularly useful comment broke down the physics: a MIG puddle is a tiny pool of liquid steel saturated with dissolved gases. While the arc is active, gases keep getting pushed out by stirring and shielding. The moment you stop, all that motion ends — and whatever gas is left has milliseconds to escape before solidification traps it.

The practical fixes are simple and worth memorizing: pause the gun in place after releasing the trigger, use a back-step or whip at the termination, and clean the metal more aggressively than you think you need to. A welder also noted that if your machine has a "crater fill" setting or burnback adjustment, this is exactly what it's for.

This is a perfect example of why r/Welding is valuable for new tradespeople — a beginner gets specific, mechanical explanations from people who've solved the same problem hundreds of times.