I have an old green Stanley metal thermos, and it is rusting on the bottom (which appears to be chrome). The inside of the thermos is fine. How do I stop it from rusting further? Is there something I can coat the bottom of it with, like Rustoleum, or some kind of a plastic/rubber cup that fits the bottom? I set it on the counter and there is usually a little ring of rust left when I pick it up, or flakes of rust if the counter is dry. Thanks for any help!