If you have a very old piano, or simply a piano that hasn't been well taken care of, you may notice that the formerly white ivory keys have turned yellow over time. Cleaning the keys can reduce the yellow stains, but probably not permanently reverse the staining. It may take some time to make your keys look a little brighter, but restoring the beauty of your piano is possible with frequent cleaning and polishing.
Dilute 1/2 cup of vinegar in 2 cups of water.
Apply the mixture to a soft, line-free cloth, but do not saturate the cloth; dampen it, or water could leak into the keys and ruin them.
Wipe the damp cloth. Go back over the keys with a dry cloth until they are completely dried off so no moisture remains on the keys.
Scrub each key with regular white toothpaste and a toothbrush.
Massage the toothpaste away gently with a slightly dampened soft cloth.
Leave the keys uncovered overnight.
Clean the keys once a week and over a period of time, the yellow keys will begin to whiten.