Window designs have evolved over centuries to enhance the beauty of a building, keep out the storms, let in the breeze, and give you a view of the trees, the moon, and the universe beyond. Not to mention, they’ve held up to decades of use and abuse. But with a round of maintenance and a few repairs, old wood sash windows will easily last another century.
Window glazing is a combination of science and art that can only be developed with experience. As a beginner, start with sash from the barn or back room, where your early results will be good enough as you improve your technique. Once you have done 10 sashes, trying to do better each time, you’ll be more proficient. By the time you get around to the front windows, your glazing will look just fine.
Step 1: Pre-treat, if needed
This step may not be needed if all wood surfaces are perfectly sound. If the bottom edge of the lower sash’s bottom rail is in good condition, it doesn’t need treatment since it has done well bare for so many years. However, if it shows signs of water deterioration, give it a coat of pre-treatment.
When you apply a pre-treatment to the bare wood [A], put it on the sash (both faces), muntin bars, and muntins. If you are using consolidating oil, put it in the glazing rabbets, too, and also add extra at each joint so it soaks in and treats the end-grain within. (Be careful not to apply pre-treatment to the side edges of the sashes and top edge of the upper sash.) Let the treatment dry before proceeding.
If you’re not going to pre-treat the entire sash, apply a consolidating oil or linseed oil mixture to the glazing rabbet. This prevents the wood from absorbing too much oil from the putty, which can lead to putty adhesion problems.
Safety Tip: Old windows usually contain lead paint—always follow lead-safe work practices. For more information, visit epa.gov/lead.
Step 2: Sand wood surfaces, if needed
All surfaces and arrises should feel smooth. Sand off nibs and whiskers, then dust with a HEPA vacuum and tack cloth.
Step 3: Prime the sash
Prime all the surfaces of the sash except the side edges (that run in the tracks) and the face margins. Do not prime the glazing rabbets if they were pre-treated with an oil-resin. Allow to dry thoroughly.
If you are priming the glazing rabbets, start by brushing a little puddle of primer on at each joint [B]; it will soak into the joint and help seal the end-grain. Leaving some primer puddled on the surface at the joints also makes it easier and quicker to get primer into the corners when you come back to brush the primer out across the surface.
Step 4: Sand primed surfaces, if needed
As above, sand off any nibs and whiskers until all surfaces feel smooth. Dust with a HEPA vacuum or brush and tack cloth. Re-prime any spots or areas of bare wood where sanding removed the primer.
Step 5: Bed the pane
Start by making sure all the panes fit by laying them in place in the sash. When you take them out, keep them in an order you can remember so they go back where they belong. (I lay the panes out on the bench in the same arrangement; some stick on blue painter’s tape and number it.) Warm up the putty by kneading it in your hand, then press the putty into the glazing rabbet with your fingers, thumb, or the heel of your hand [C].
Next, lay a “bed” of putty in all the glazing rabbets of the sash. Place each pane of glass on the bedding putty, making sure the bottom edge of glass rests on the neck of the lower glazing rabbet.
Jiggle the pane slightly with your fingers along the edges so it beds down into the putty. Leave at least 1⁄16" of back putty between the glass and the shoulder of the glazing rabbet. You should have some putty squeezing out along all the edges of the glass.
Panes wider than 24" may require spacer blocks between the edges of the glass and the neck of the glazing rabbet—little pieces of wood or plastic that help secure heavier panes until the putty firms up.
Pro Tip: You can quickly “bed” the pane down into the putty using an electric palm sander with a rubber pad (no sandpaper). Only use a new (low-cost) palm sander to avoid scratching the glass with any residual grit. Turn the sander on, and touch the rubber pad on the glass for a quick second or two. Be careful; it’s easy to squeeze too much putty out of the bed—leave at least 1/16” of putty between the pane and the shoulder of the glazing rabbet.
Step 6: Set glazing points
Set at least one point per edge on panes smaller than 6" or 8". On larger panes, set points away from the corners of the glass at least one-fourth the length of the edge, and then every 8" to 10" in between.
Lay the point flat on the glass with the sharp end leading into the neck of the glazing rabbet [D]. Set the edge of the putty knife across the tabs and wiggle it side-to-side, pushing the point into the wood until the tabs are flush with the neck of the glazing rabbet [E]. (The wiggle helps ease the point into the wood with less stress on the glass.) You may have to hold thin muntins from behind to keep from breaking the rib or muntin.
Pro Tip: You can set points much faster with a point driver. Load the magazine with diamond or triangle points, set it gently onto the pane, and squeeze the trigger—the spring-loaded mechanism shoots a point into the neck of the glazing rabbet. Be careful to squeeze the trigger up, and don’t push down on the handle (as you would with a staple gun), which can break the glass.
Step 7: Tool the face putty
Go around the entire sash, placing and packing lines of putty; it doesn’t have to look good at first. Some people roll the putty into little snakes—you can do this, but it takes extra time. I like to quickly distribute the putty using the palm of my hand or a putty knife.
Next, pack the face putty into place—put the end of the putty knife on the pane and wiggle the knife slightly to force the putty into place [F]. It still doesn’t have to look good.
Finally, tool the surface of the face putty using the “end edge” or “in line” technique (see below) to form an even bevel that looks good and packs putty firmly into the glazing rabbet, forming a good seal all along the glass and wood. You should be able to look through the pane and see that the edge of your putty is directly across from the arris on the other side of the glass. Form neat miters at the corners, where the bevels meet.
Tooling Technique: End Edge
Keep the edge of the putty knife on the glass through most of the stroke. This technique leaves no putty on the glass, which saves time. Hold the knife at a slight angle to compress the putty into the glazing rabbet during the stroke, leaving a smooth, flat bevel of putty. Stroke slowly to allow the putty to “flow” onto the underneath side of the knife.
Tooling Technique: In Line
Hold the knife at a slight angle so the putty compresses into the glazing rabbet and leaves a smooth, flat bevel of putty. This technique leaves a little putty on the glass that you will have to go back and remove. I like to do so by taking my warm wad of putty, forming it into a ridge, and wiping it along the edge of the pane, which picks up the waste putty.
Step 8: Polish and clean the pane
Immediately after tooling, you’ll need to polish the outside of the pane with whiting to clean oil from the putty off the glass and “dust up” the putty’s surface to promote drying.
Tamp a dry, soft-bristled 3" or 4" paintbrush into the dry powdered whiting and work a small amount into the bristles. Starting at the top of the panes, gently dust the surface of the glass with the brush [G]. Work down the panes to remove all the whiting and oil from the glass surface. It’s OK to brush the surface of the putty, but be careful to not jam the ends of the bristles directly into it.
Sweep any remaining buildup of whiting out of the lower corners, and finish with wide, sweeping diagonal strokes, which leaves the pane clean, clear, and free of oily fingerprints and whiting.
Safety Tip: When polishing with whiting, wear goggles and a respirator to keep it out of your eyes, nose, mouth, and lungs.
Step 9: Tool the back putty
Flip the sash over, and use a putty knife to trim off the excess putty that squeezed out from the bedding [H].
Step 10: Polish and clean the panes
“Polish” the inside of panes with whiting, as in Step 8. Then set the sash aside to let the putty cure. (Cure times will vary depending upon the putty used and the environmental conditions.) Place the sash in correct vertical position, exterior side out, and lean it slightly back to avoid distorting the putty bevel.
Step 11: Prime the putty, if needed
Some putty and paint combinations need to be primed; others don’t—check the manufacturer’s recommendations. Apply primer to face putty bevels and interior seals, lapping it 1⁄16" onto the glass. Let dry.
Step 12: Paint the sash and putty
Brush two topcoats of paint onto the entire sash (except the side, top, and bottom edges if they’re not being painted), lapping paint 1⁄16" onto the glass, which seals out rainwater and extends the putty’s service life. Use high-quality enamel or exterior house paint. Waterborne 100-percent acrylic paint is good, as are oil-based alkyd resin or linseed oil paints.
If you use acrylic house paint, be sure to take a rag and wipe off the side margins of the sash [I] and the meeting surfaces of the meeting rails. This will help prevent the sash from sticking to the sash track and stops.
Online Bonus: Window Repair Video
See Part 2 and more videos at Historic HomeWorks.