You can buy the black paintable fender welting from pretty well any hot rod shop. All 3 big car manufacturers used it with bolt on fenders, mainly to keep splashed up muddy water from flying out all over the sides of the car at the bolted joint. It also cut down on squeaks and made for a less finicky fit between fenders and main body. The older cars in the twenties and thirties and early fourties used a welting with a round bead on it, some of the later fourties and early fifties used a flat welting with no exposed bead. You used to be able to buy a vinyl welting that had a chrome like finish on the exposed bead.