If your welding has been under-sealed no need to touch it im assuming these are manufactured floorpans painted normally, a couple of coats of Hammerite will see your car out if its just been plated check beyond the weld see what condition the original metal is like any rust treat it with the acid or chemical rust eater {they really dont destroy the rust }.follow the instructions when bone dry hammerite it or a galv spray if you want to do a quick job