Toss the blue tape. Your number-one tool is a 1½
- to 2-inch sash brush. Its angled bristles come to a point, giving you a fine line. Raise the bottom sash and lower the top sash so they've almost switched places.
1) Paint the exposed parts of the top sash (now on the bottom), including the muntins.
2) Carry a thin line of paint onto the glass to seal the glazing. Next, nearly close the window and
3) paint the rest of the top sash, as well as
4) the entire bottom sash, without getting paint between the sash and the stops (the pieces of wood in front that hold them in place).
5) Then paint the casing, sill, and apron. Before the paint dries, move the sash up and down. "If you can't see a clear crack between the sash and the stop because of wet paint," says Tom Silva, "then you just glued the window shut."
See a more detailed step-by-step at How to Paint Doors, Windows, and Walls