I would only apply por 15 to rusty surfaces, it does not adhere to clean metal.What I usually do whenever all the carpets are out is to apply a coat of POR-15 to the entire floorboards, trunk, wherever water can cause mischief. Because if it is not leaking now it probably will down the line. This way I am protected for when the inevitable occurs. As we all know leaks are never obvious at first. They begin small and the carpets get wet, hold onto it, and the rust begins. We find it out too late so a coat of rustproofing when the metal is sound is just good insurance.