Pianotech

B72 is soluble in acetone and in lacquer thinner. It is also soluble with difficulty in alcohol. The reasoning behind using B72 dissolved in alcohol is to have it penetrate the felt, and not flash off as rapidly so as to draw more solids on the surface of the hammer.
The solids in B72 are an acrylate, very similar to what we often call “keytop,” but is really ground plexiglas, another acrylic polymer. (Actual keytops have also been used, but there are lots of different formulations, and many are far from ideal for the purpose). I doubt there is any perceptible difference between ground plexiglas and paraloid B72 in behavior relative to hammer fibers, if both are applied dissolved in the same agent (acetone or lacquer thinner).
I’d suggest having the thinner cover the felt to the thickest part of the shoulders. It will penetrate (wick in) farther.

Very cool! Does it work with B72? How deep do you go into the tray? I guess you stop before the molding.