72 is a nickel-iron non-conductive electrode formulated to join dirty and contaminated cast irons. This universal electrode can be used to weld a wide variety of malleable, ductile, and nodular cast irons. In addition, it is ideally suited for the repair of defective or cracked malleable iron castings. 77 cast iron welding rods are softer than nickel rods, and have unique properties that allow the cast iron welds to stretch and elongate up to 300 percent more than other rods, which helps prevent the base metal and weld from cracking in the process. Since 77 has controlled penetration, which is ideal for non-cracking and machinability, it may not be able to bond exceptionally dirty cast iron. 72 should be used for this application. It has a high penetration, can anchor deep into the subsurface of dirty cast iron, and seals in porosity generating contaminates prior to finish welding with 77. 72 is used for a cladding operation, then the weld is completed with 77.