Painting the overlapping sash on double-hung windows requires a bit of artful sliding to efficiently reach all the parts that need painting. For a good-looking paint job with a minimum of fuss, follow these steps, recommended by painting pro (and TOH contributor) John Dee of Concord, Massachusetts.
Lower the upper sash. Unlock the sash and push the lower sash up about two-thirds of the way toward the top of the window opening. Pull the upper sash about two-thirds of the way down. Paint the muntins (if any) and the faces of the rails and stiles of the lower sash that aren't covered by the upper sash (1). Also, paint the bottom of the upper sash's meeting rail. Leave a narrow, 1/16-inch bead of paint on the glass to seal the edges of the panes. As you coat the stiles, wipe up any paint that gets between the sash and the stops so the sash won't become stuck when the paint dries.
Switch sash positions. Immediately push the upper sash about two-thirds of the way up into the opening, pull the lower sash about two-thirds of the way down, and finish painting the lower sash (2). Paint the rest of the upper sash, starting with the muntins. Do not paint the jambs or the stops.
Keep 'em moving. Go inside and wipe any wet paint off the inside of the lower sash's top rail. Then as the paint is drying, slide both sash up and down a few times so the paint doesn't glue the windows in place. Wait at least overnight before shutting the window completely so that the paint doesn't stIck to the wrong surface.