4 To prevent future rust, and if your car is a long-term project that will span months or even years, it’s best to protect your new metal with some sort of weldable primer material. Don’t make the mistake of using POR-15 or similar products on surfaces to be welded. POR-15 is an insulator and can’t be welded through. For our panel, we sprayed the welding surfaces with this copper weld-through primer then covered the non-welding areas with a red oxide primer. You can also use brush on zinc primer, which will protect metal from rust and maintain a conductive welding surface.