For ductile iron, use Direct Current (DC) for welding because it produces less spatter and provides better penetration. But, an Alternating Current (AC) setting is preferred for brittle grey iron because it can form a stable arc and prevent cracking.
When welding cast iron, the direct current should be used with as low an amperage setting as possible to reduce porosity and spatter levels.
A 1/16-inch diameter electrode is ideal since it can produce small beads and deep penetration. Plus, adding a small amount of Reverse Polarity (RP) may be beneficial if some extra penetration is desired.