It’s best to remove the sash from the frame before reglazing and painting it. This will prevent the window from being painted shut and give you more control during the glazing and painting process.
If any of the glass isn’t sitting flush in the glazing rabbet, try to remove the glass and clean out the gunk. If any of the muntins are warped, place a caul under them to bring them up to the surface on the glass (bend the wood, not the glass.) You can also apply a thin bead of glazing compound in the glazing rabbet to level the glass.
Use fresh putty. The best way to do this is to buy it from a place that sells a lot of it. Squeeze the outside of the container. If it is hard, skip it. Conversely, if it is too juicy, also avoid it. The consistency of glazing putty changes with varying temperature and humidity conditions. You’ll have to experiment with this to find the ideal environment for glazing.
If you are spot-glazing a sash that is vertical in a frame, use a 1” putty knife to press tablespoon sized globs of putty into the glazing rabbet. Then, position your putty knife in a corner to form the correct angled bevel and draw your line making firm contact with both the wood and glass. Do this in one continuous motion. Remove the excess putty. (If the sash is horizontal, roll the putty between your hands to form a cylinder of putty and press this into the glazing rabbet.)
If the putty sticks to the glazing knife, dip the knife in the solvent appropriate for that putty (mineral spirits for Dap-33.) If you use oil, you’ll end up with a gooey mess that will take forever to dry.
Don’t try cleaning the glass of putty residue until after the paint has dried. If you must do this, be careful not to smear the fresh putty. Use regular window cleaner (e.g., Windex) to clean up the putty residue. You can also use whiting, which is available at stained-glass supply stores. Use a soft, dry brush to lightly brush the whiting against the wet putty. This will help to accelerate the curing process.
The glazing process is not complete until the putty glazing is painted. It is crucial that a narrow band of the glass be painted, along with the putty and wood, effectively forming a cohesive blanket that unifies these elements. The purpose of painting the glass is to keep moisture from infiltrating and subsequently eroding the putty.
I like to use a 1″ sash brush to paint window sashes. Make sure your paint is viscous enough to put down a nice thick coat, but not so thick that you leave brush marks in the uncured putty. (If time permits, allow the fresh putty to skin-over before applying paint. Letting sashes cure in a hot and dry location will expedite this process.) If you’ve used Dap-33 putty, be sure to use oil-based primer. Dip the brush in the appropriate solvent (usually mineral spirits or water) for the paint you are using. Then wipe it off so your brush is moist, but not wet. This will make cleanup a lot easier. Start by painting the glazing (or inside profile.) Then move to the rails and leave the long wood grain of the stiles for last. Don’t forget to paint the tops or bottoms of the meeting rails.
If you are using masking tape, be sure to remove the tape while the paint is still wet. You’ll have to reapply tape for the top coat.
Paint the bottom ends of the stiles on the lower sash to prevent capillary action from drawing moisture up into the end grain of the wood. This is a major source of checking and rot and by blocking the end grain with paint, the life of the sash will be prolonged. If, after priming, there are still large visible checks on the ends of the stiles, caulk them up. Also caulk any gaps on the outside sash face between the rails and stiles. For gaps larger than 1/8″, you may want to consider rebuilding the sash.
Don’t paint the bottom side of the rail on the lower sash or the jamb side of the stiles. When these get wet, an unpainted side will help them dry out. You may also want to avoid painting the wedged shape portion of the meeting rail, but be sure to sand it smooth so it releases properly after being pressed against its mate.
Paint the outside of the sash before painting the inside. Dry paint on the fresh putty will help protect the putty from becoming damaged.
One way to paint both sides of a sash during the same session is to paint one side, flip it and set the glass on on a 2″ thick piece of styrofoam slightly smaller than the size of the glass. (This only works for single, not multi-light sashes.)
If you can avoid it, don’t paint the parting strip – especially the edges, as they come in contact with the sashes. If you must paint the parting strip (for cosmetic purposes,) apply a thin coat that will dry to a smooth and hard finish. After the paint has fully cured, sand it with fine sandpaper and wax it so the sashes glide smoothly against it. If the parting strip is thick with layers of paint, you may want to consider replacing it.
Try to get as much old paint as possible off of the inside edge of the blind stop. This will help the upper sash lower all the way. Likewise, sand smooth the inside edge of the inner stop.