I am brainstorming on get rid of some nasty rust in front of the back tires and behind the front tires and along the rocker panels...then instead of painting, thinking about polyurethane (spray-in boxliner stuff) sprayed from the door trim right down to the rockers. This is '91 Explorer in pretty good shape, but I'm not concerned about perfection. Will Poly hold onto body filler (if I go that route)? Is Poly expensive?