Like cuber said, this is typical lacquer wear. It's caused by the acids in your saliva and the oils in your skin reacting to the brass. Take some baking soda and add water to make like a paste. Dab a little on the horn on the bad areas. DON'T SCRUB IT! Scrubbing it may further damage the lacquer. Just wipe it on, leave it on for a couple minutes and wipe it off. This should lighten the spots. Be careful not to get the baking soda paste around the pads. It can really make a mess. Jim