There are several good products used as mildewcide that will kill mycelia (the fuzzy growth) as well as spores (that regrow new mold when conditions are right). They are sold in Home Depot and paint stores under several brand names- Krud Kutter, MX-3, M-1. They are sold in small packets of liquid or small bottles to add to a gallon of paint. You can mix a packet with water or diluted alcohol and spray it on the hammers. It might help voice those old hammers too. : -) These should last for at least a couple years. Maybe a Damp chaser heater stick would be a good idea too. Douglas Gregg Classic Piano Doc
This PTG archive page provided courtesy of Moy Piano Service, LLC