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You’ll also need filler rod to match the metal you’re welding, and it’s also available in several different diameters. Use ER70S2 on mild steel and chromoly, 312 on stainless steel, and 5356 for either 3006 or 6061 aluminum alloys (you can also use 4043 on aluminum but not on parts that will be anodized, as those will blacken in the anodizing process). Though there is a little fudge factor, it’s a good rule of thumb to use a filler rod that’s about the same thickness of the material you’re welding. When in doubt, 3⁄32 is a good size.