From DIYinfo.org
It nearly always takes more time to paint a window than you think, because of the number of surfaces and because you have to keep paint off the glass. For security reasons, you will probably want to close windows at night, so start work as early as possible.
[edit] Protecting the glass
If you use masking tape to keep paint off the glass, set it back by about 3mm so that a thin line of paint goes onto the glass. This will seal any gap between the glass and the frame. As an alternative to tape, use an aluminium masking shield, moving it along as you paint. Clean the shield regularly as paint tends to creep under it.
1. Open the window. Paint the frame in the order illustrated with a 1in or 2in (25mm or 50mm) brush or an angled cutting in brush. Do not apply too much paint in one coat or it will run and take longer to dry.
The painting sequence is largely determined by the fact that the brush strokes should follow the construction of the joinery; so the vertical brush strokes will 'cut off the horizontal ones.
2. Keep paint off handles and stays. These look best cleaned up and left natural. Remove any dried paint splashes on metal with wire wool or a wire brush lubricated with white spirit.
3. If you have to close casements and the paint is touch dry but not absolutely hard, rub a little talc on the meeting surfaces. Alternatively, place a sheet of plastic kitchen film between surfaces likely to stick.
[edit] Paint casement windows
1. CrossÂbars and rebates
2. Top and bottom cross-rails
3. Hanging stile and hinge edge
4. Meeting stile
5. Frame
[edit] Sash Windows
1. Pull the bottom sash up and the top one down, so that you can paint the meeting rails.
2. Paint the frame following the order illustrated below. You almost need to close the window to paint the inside runners. Give the runners a very thin coat to prevent the surfaces from sticking.
3. Do not let paint get onto the sash cords or they will harden and fail earlier than they otherwise might.
Open sash windows and reverse their positions, then paint them in the following order:
1. Meeting rail
2. Vertical bars as far as possible
3. The area that the inner sash sits on, and lower runners
4. Cross-rail and underside. Reverse the windows
5. Cross-rail
6. Vertical bars
7. Cross-rail
8. Rest of vertical bars
9. Soffit, top runners and behind cords
10. Frame
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