Typically it is with older instruments.If your instrument has a coat of lacquer it will not happen. However, it is when you have direct contact with the raw brass that the reaction occurs. It is simply a dezincification process and is a normal reaction when your sweat reacts with raw brass.
For the earlier poster (And anyone else (Band Directors) who might want to improve the image of and longevity of your instruments). If you have school instruments that lacquer has come off you can do some simple refinishing yourself.
This is for older school instruments. This is NOT intended for high quality professional type instruments.
!!!! THIS IS ONLY TO BE DONE WITH THE BAND DIRECTORS PERMISSION !!!!!
Clean the exposed area with DAWN dishwashing detergent (use the original orange colored one not the scented ones).
Dry the area completly.
Polish the area with Flitz or Maas.(Brand name brass cleaner) DO NOT use Brasso or any other coarse polish.
Then clean and spray special lacquer on it - you can get is from Voltaw tools.
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http://www.votawtool.com/zcom.asp?pg=products&grp=745)