Cornies may be an exception, I haven't tried on one of them yet (one day I'll get a kegging setup), but for polishing metal I use some #0000 steel wool. It works great for getting rust & petinia off most metal surfaces, and being extremely fine, it doesn't scratch. Just lay down a thin layer of oil (3-in-1, or WD-40 might work), and give it a light scrubbing.
I also use it for delabeling bottles. After a soak in oxyclean, and a quick scraping with a razor, any residual glue comes right off...
Just my $0.02, if someone feels like trying it on a corny, lemme know if it works...