How to prepare wooden frame sash windows for painting
It was very important to paint the wooden frames of sash windows regularly. Otherwise old paint pealed off which looked unsightly and was impossible to paint over neatly.
Also damp would get in which, in the case of sash windows, would swell the wooden frames. This made the windows stick and even lock in position so that they couldn't be opened or closed.
Painting sash window frames, though, was quite a palaver.
An aside on metal casement windows
Metal window frames seem to have been common in the 1940s and possibly before for factories. They also had to be painted regularly because once the damp got in the frames rusted.
Difficulties of access for painting
One reason why painting was such a palaver was that some part of the window and the wall frame which held it was always inaccessible, however the window opening was adjusted. The task tended to be require alternatally being inside the house and outside up a ladder. At worse the window needed to be partly or wholly removed - see also mending sash windows.
Difficulties of building up layers of paint
Another problem was that however carefully one painted, some paint invariably spilled into the channels. So layers of set paint built up there over time which spoilt the fit and made the windows stick. As often as not, windows simply would not open. Consequently care of sash windows was a vicious circle: painting was essential for maintenance purposes, but the more often one painted, the more the window tended to stick.
Difficulties of aging putty
It was also near impossible to get a neat painted edge on the glass. The panes of glass were fixed into their wooden frames with putty. This, when first used, is a pliable material based on linseed oil, which sets hard on exposure to the air as the linseed oil evaporates and seeps into the wood. Then it is waterproof.
Over time, however, putty contracted, and small cracks developed which progressively became deeper. Eventually pieces broke off. Painting over the putty while painting the wooden window frame prolonged its life, but in time the edge that met the glass became bumpy and jagged, so letting water in, as well as producing a messy appearance. People usually tried to avoid removing the putty and replacing it with new, but eventually it really needed doing. In practice, they all-too-often did a botch job or just left things as they were to get progressively worse.
I never lived with sash windows, but I do remember as a child visiting houses which had them and being told, "That window doesn't open".