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So what happens when you accidentally touch the tungsten to the metal? You get a nice glob of metal on the end of your tungsten that needs to be ground away. Note the blackish/ brown cloud around the weld and porosity; that’s what it looks like if you try to weld with tainted tungsten. Also, though most welders skip it, it’s good practice to clip off the end of a used filler rod (ours is 1/16-inch ER70S-2 for mild steel) to prevent contamination—just like in MIG welding.