When MIG welding, always use a smaller diameter wire for thinner metals and larger wire for thicker metals. Thick metals also require a larger machine. Make sure that the electrode extends from the tip of the contact tube by at least 1/4 inch, and always use the appropriate shielding gas for the type of metal being welded.
When penetrating welds on steel, CO2 is a good shielding gas. Use a combination of 75 percent argon and 25 percent CO2 for thinner metals and steels. Different types of wire are needed depending on the base metal you are welding. Use stainless steel wires when working with stainless steel and aluminum wires for an aluminium base. Keep the wire pointed towards the leading edge of the weld pool to maintain maximum control of the weld bead.
Always keep the gun as straight as possible, using both hands to keep it steady. Clean the gun liner and drive rolls from time to time, and always match them to the size of the wire. Keep the wire in a clean environment when not in use, as contaminated wire can result in a poor weld. For a narrower bead, use a pull-gun technique, and use a push-gun technique for a wider bead. Make sure that the parts being welded have the same thickness as the leg of the weld.