I spent about two hours with various tools trying to pry loose the
upper sash of a relatively new double hung window in my mother's
bathroom. The window is circa 1973 and with an aluminum sash channel,
so there's no parting stop to remove and consequently no possibility
to take out the lower (or inner) sash in order to get at the outer.

All I managed to do was begin to separate the pane from the lower
horizontal wood strip of the sash, from hammering. Ultimately it
seems the dude who called himself a housepainter painted the window
shut from the outside (this window is 25' above the ground).

Is there any fixative or paint remover that comes in a spray can, or
with a tiny "straw" attached to it like you find on stuff like WD-40?
Aside from totally removing the window, can anyone think of a way to
loosen this sash?
The whole problem came to my attention when I realized how filthy the
windows were and tried to wash them.