Not sure what you are trying to build but I would do a few test pieces. Lis2323 mentioned grinding to shiny metal and I would start with a 5P+. If that's not satisfactory talk to your local welding shop or supply store. They usually have some good suggestions.What is the best way to weld up Black Iron? Can brake or carb cleaner be used to get it clean enough to weld and what is the best rod for it. I weld on rare occasions just for home projects..
That or use the wire wheel on the bench grinder to get it shiny, otherwise whatever rod you are used to should be fine. Its called iron pipe, but the pipe is actually steel, not iron (fittings may be iron though).I would just grind the weld area (and a spot to attach your ground clamp) shiny.