You need to get the horrible peeled up brittle glossy top coat
off, which isn't a nice job.

Oh dear Christian, you seem to have encountered a case of
poor preparation/practice in painting.

Yes, I do like the punishment of buying ex-student terraces that haven't
been repainted in decades. At some point I really do need to renovate the
windows, which will be a complete removal, strip down, draughtproofing and
painting. I might still use a bit of solvent based paint on the window
sashes for its greater mechanical resistance where it slides, but I might
not.

Christian.