I am looking at a Getzen Severinsen that is so tarnished it's almost uniformly black. I have read and seen pictures of folks doing the salt/baking soda tin foil water bath and getting amazing results. Is there a level of tarnish so severe that the finish might be damaged and I should be concerned? Or does that layer of tarnish uniformly covering the entire horn mean yay! the silver is probably intact? The case is in decent shape, the slides are free, the valves still move smoothly, but that finish - ugh! Thanks!