Painting windows requires special preparation and techniques.
To protect the glass, you can use masking tape or a wipe-on protective film that's dispensed much like deodorant. Another option is to skip this step and scrape off any paint from the glass with a razor. It's a case of whether you spend your time masking or scraping.
Apply the tape or film only after thoroughly dusting the corners of each window pane. The crevice tool of a vacuum works well to do this.
If you paint the windows without tape or film, you can use a trim guard to protect the glass area. These come either as a triangular-shaped piece of metal or a metal strip with a plastic handle. On bare wood windows, allow some paint to seal against the glass.
When it comes to removing hardware, purists will tell you to remove it before painting. But this is not always practical.
If a window lock is covered by several layers of paint but works properly, decide how much time you want to spend on the window. You will crack the existing paint if you remove the lock. You also leave a dent in the paint the shape of the lock. Unless you replace it with the same type, you will have to sand away the paint buildup in this area. If the hardware is not painted, it may be easier to remove it rather than paint around it.
Paint windows from the inside out. Begin painting the thin vertical and horizontal dividers between the panes in double-hung windows. The same holds true for casement windows.
If your window has a removable window grille, take it out for a much easier job. Use a 1 1/2-inch-wide sash brush, and don't drag your brush back across the edge or it will cause a run in the paint.
If you are painting a double-hung window and the upper sash is movable, reverse the position of the inner or lower sash and the outer or upper sash. Paint the lower half of the outer sash first, then paint the inside sash. But don't do the top edge where the lock is. Save that until last.
Return the sashes to their normal positions, but don't close the window completely. Then paint the top of the outer sash and the top edge of the inner sash.
Switch to a 2 1/2-inch-wide brush and paint the window frame from the inside out moving the sash cord, if there is one, out of the way to avoid getting paint on it. Next, paint the window casing. Cut in a nice clean line where the casing meets the wall. Then paint the sill and the apron, which is the trim beneath the sill.
When the paint is dry, scrape it off the glass with a razor scraper. Run the scraper down the joint between the glass and frame, then scrape away the paint working toward the sash.
Of course, if the upper sash is painted shut, you can't move it, so paint the upper sash from the inside out, then open the lower sash and paint it. Leave it slightly open so the paint will not get dirty at the bottom of the sash.