No welding rod of any kind. The smallest wire-feed welder will burn a gigantic hole in it. Even with the older radiators with the brass tanks, brazing would melt them. I've had good luck soldering the older brass cooling tubes but you have to get them spotlessly clean first, then use a micro-tipped acetylene torch. Propane usually won't get hot enough. If your radiator has aluminum cooling tubes, just give up and buy a new one. Especially if the hole was caused by corrosion. Once that starts, you'll be chasing holes forever.