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Arc welders use a variety of different electrodes, also referred to as welding rods. Choosing the right welding rod will determine the how easily the weld is performed, and how strong the weld will hold. If the wrong welding rod is chosen the weld might not work at all. Keep in mind welding takes time to learn, and even though the right rod may have been chosen, it still will require skill to get a proper weld.
Determine the type of steel you are welding. Standard iron will require a different rod than stainless steel or cast iron. Stainless steel requires a specific rod for that type of metal, usually 308L. In most cases a 6013 electrode will work for most steels. 6013 is a general use electrode. NI-CL is used for cast iron.
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Measure the thickness of the metal you are welding. This will determine the diameter of the welding rod that you use. The diameter of the welding rod should be half the thickness of the metal you are using. The rod can be smaller, however it will require more than one pass to complete the weld.
Check to see if your arc welder runs on DC or AC electricity. Certain rods are used for one or the other, or both. DC rods tend to have deeper penetration and are more difficult to get started. However the welds can be done quicker when using DC specific rods.