I'm starting to rebuild a Yamaha with silver plate. Part of this rebuild is going to be a thorough cleaning. Most spots on the body that are under a key or rod have a black gunk on them -- not tarnish, though there is tarnish under it when you remove it. I've tried to clean a small area with alcohol to test. It did an OK job, but it took too long to be effective for the whole body. What have you guys used to clean similar junk?