There are some old pianos here in Canada that have a crackled aligator skin type of finish on them and these will leave a prominant whitish imprint of your butt if you perspire on them. in fact, I think part of it is to do with heat also because a dry hot coffee cup will do the same thing. Sometimes the whitish mark dissapears after a while, on some others it can stay there fairly permanent if the object is hot enough.
It is the heat that makes the old finish soft enough for it to grab moisture out of the air causing the whitish cloudiness.
Spray a light coat over top and this melts the surface momentarily and allows the moisture to escape previous to the finish setting hard again.
Wiping a methyl hydrate soaked rag over top of old lacquer or shellac will do the same thing. The surface melts for a few seconds, grabs moisture out of the air and sets leaving whitish streaks in the finish. Spray a top coat and the white disappears.
That is, on Nitro (well known for the whitish clouds) , or are PU also sensitive ?
For the Nitro, special formulas are sold, which seem to be a mix of NItro solvent and alcohol, they do what you say, melt a little the surface and allow it to tense back (sometime)
DO that on shellac and you will ruin the surface ("burn" the shellac) On alcohol based products only alcohol(s) may be used. (you will not find a piano bench French polished, however)
