Strictly from a chemistry standpoint, the old standby weak vinegar solution should work. May take more than just one swipe, but the acid should gently remove the mildew, without adding the sufactants from Pine-Sol. The trouble is going to be KEEPING the mildew gone. You could instruct your clients in the proper technique to periodically clean the soundboard. If you have "chemical" friends, there are some things that could be used to permenantly kill the little buggers. Sodium azide, for one. Highly effective in extremely dilute solution, so no damage to the finish. Problem is (of course) it is also extremely toxic, and, in solid form, explosive on contact with heavy metals. Not for the faint-hearted. Another option is the ever-present (and daily-ingested) BHT. You have to use a little more, but it'll work. Again, it's not going to peel the finish from the soundboard. Is this enough of a problem, generally, that someone should market certain chemicals specifically to piano techs?
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