G. Morgan wrote in news: snipped-for-privacy@Osama-is-dead.net:
It all depends on what the ring is. If it is grime and dirt, soap and a brush should get it. If it is rust (not uncommon), then strong HCl (also called muriatic acid) is one possible treatment. Another is oxalic acid solution with a bit of vinegar or HCl. The reasoning is that FeCl3 is nicely soluble in water, but FeCl2 or rather Fe(OH)2 is not. That is the first step in the formation of rust. That means you want to oxidize the Fe2+ to Fe3+ and have chloride ions there to help dissolve the Fe3+ ions. Oxalic acid is an oxidizer, moreover, it is a chelating agent (it sort of envelops each Fe3+ ion, and keeps it in solution). Keep it acid to prevent reduction to Fe2+ and Fe hydroxide and rust formation.