Jerry has perfectly described the correct method for welding cast iron with an electric welder. The peening is absolutly required and as with all welding the proper rod is esential. Being an old guy I still sort of prefer gas welding, i.e., brazing on cast iron. but I've used DC nickle rod with success too. Forget MIG wleding.
It has been mentioned above but getting it really clean is absolute with either method.
