Start with the material, not the machine
The base material sets the boundaries before anything else. Thin aluminium and stainless are heat-sensitive and want the tightest possible control, which points at TIG. Heavy carbon steel with mill scale in an open yard wants a process that tolerates contamination and wind, which points at SMAW. Long runs of clean structural steel where speed matters point at MIG.
Guessing costs money in both directions. Use TIG on a hundred feet of heavy fillet weld and the labour bill is indefensible. Use MIG with solid wire on aluminium sheet in a breeze and the weld is porous before it has cooled.
Then thickness and position
Thickness determines heat input and whether multiple passes and a bevel are needed. Below roughly 16 gauge, burn-through becomes the main risk and TIG's independent heat control wins. Above about a quarter inch, joint preparation and pass sequencing matter more than the process badge on the machine.
Position matters just as much. A joint that has to be welded overhead against the underside of a machine frame narrows the options fast, because puddle control fights gravity. That is often where stick electrode is genuinely the most practical answer, not a compromise.
Conditions on site change the answer
In a shop, gas-shielded processes are the default. Outdoors in Nebraska, wind is the deciding factor. Shielding gas is what protects the molten pool from the atmosphere, and even a gentle breeze can strip it away and leave porosity that is invisible until the weld fails.
That is why an honest mobile welder carries multiple processes. Either the joint gets screened and enclosed so gas-shielded welding is viable, or the process changes to self-shielded flux-cored wire or stick electrode where the shielding travels with the consumable.
Where oxy-acetylene still belongs
Oxy-acetylene is less often used for joining these days, but it remains essential kit for cutting, heating, bending and preheating. Applying controlled preheat to a thick or higher-carbon section before welding is frequently what stops a repair cracking days later.
It is also the practical tool for freeing seized components and straightening bent steel — jobs that regularly come along with a welding repair.
The short version
TIG for precision, thin material, aluminium and stainless where cleanliness and appearance count. MIG for speed and volume on clean steel. SMAW for dirty, windy, out-of-position field work on carbon steel. Oxy-acetylene for cutting, heating and preheat.
If you are unsure, describe the part and send a photo. Picking the process is our job, not yours.
Written by Mason Pillard, owner-operator of Mobile Metals Welding & Fab.