Wear thick gloves, a MIG welding mask, goggles and thick clothing to protect yourself from flying sparks and from exposure to the argon gas involved in MIG welding.
Preheat the cast iron, to 650 to 700 degrees Fahrenheit, in an oven.
Strike the MIG welding electrode against the surface of the cast iron to form an arc. Pull the welding tool away from the surface to lengthen the arc, and hold the arc in place to create a welding puddle of melted iron. As the welding puddle forms, the MIG welder should be applying argon shielding gas to the surface and its vicinity to protect the weld from any oxides that might accumulate. Move the arc in a straight line to create a small weld.
Create the next weld by moving the arc to a new spot on the iron and holding it in place until another small welding puddle forms. Make small welds along the surface at regular intervals to one another, to reduce the heat-affected zone created by welding. Cover the entire length of the surface with welds, then allow the surface to cool for several minutes.
Create a second set of welds at regular intervals of distance to the first set. Allow the surface to cool. Continue to create more sets of welds until you have reached the welding integrity that you desire.
Pull the welding tool away from the cast iron to end the arc. Allow the welding puddle several minutes to cool.