Mig welding and auto body work - when to use MIG, Hammer Welding, or Tig Welding

Do you use MIG welding for auto body work? (Full credit: See this article for more info)

It’s been called a “bad idea fostered on us amateurs by our ignorance.” This is because using MIG on auto body work is akin to using a sledge hammer to hammer in finishing nails—it’s too rough for the work.

If you look at the available wire for MIG, you don't find anything much softer than S60 or higher wire. In general, the higher the yield strength of metal, the harder it is to work. So the weld bead left by MIG is extremely hard relative to body sheet metal. It’s almost impossible to work in that context. It also cracks very easily even if one tries to anneal the metal in the weld. “In restoring a 1967 Mercury Cougar I finally had to teach myself hammer welding using a welding torch,” said one welder. “The results were better!” This welder taught himself hammer welding using a welding torch. When he did…

• The seam is the same thickness as the parent sheet metal.
• The seam is as soft or softer than the parent sheet metal.
• The seam and the surrounding sheet metal are easily worked to remove any defects caused by the welding process.

Should you want to equal the hammer welding process but with an electric source of heat, TIG welding is the way to go. The filler metal selection is lots wider and the TIG can be run way down to 10 amps or so which would probably let you weld aluminum foil! Most professional welders hold the opinion that a TIG weld is superior to MIG.

So why do professional body shops use MIG? Mostly because it’s necessary for welding high-strength steel used in the structural automobile parts, but not for external sheet metals. External sheet metal—the part of the car we can see—is mild steel It’s too expensive and difficult for car manufacturers to make sheet metal forming dies with high-strength steel. Body shops replace whole panels. They rarely patch one. The high-quality restoration shops use TIG or (usually) a torch and hammer welding.