Originally posted by FreeStyle:
I put normal toothpaste (original white based) onto a damp cloth and clean the keys. It works fine so far. 
I have used diluted tooth paste to clean my customer's piano keys for the last 40 years, works fine. Do not harm the finish on a key, that finish keeps the keys protected.
First a damp cloth, if you need more tooth paste DILUTED.
John C.