Step 1: Remove the old putty
On older single-pane windows, the glass is usually surrounded by putty called “glazing compound,” which holds the glass in place and seals out the weather. This putty often lasts decades, but over the years it becomes rock-hard, cracks and even falls off the window. Loose or missing compound lets wind and rain leak in around the glass. Replacing the putty around one pane of glass will take 15 minutes to an hour, depending on the size of the pane and the stubbornness of the old putty. Replace broken glass while you're at it. This adds only a few minutes and a few dollars to the job—much cheaper than calling a glass repair service.
It's possible to replace glass and putty with the window in place, but you'll save time and get better results if you can remove the window and clamp it down on a flat surface. If you have broken glass, get it out of the way before you remove the old putty. Put on heavy gloves and eye protection, place a cloth over the broken pane and tap it with a hammer. With the glass thoroughly broken up, pull the shards out of the frame by hand. Pull out the old glazing points with a pliers. If the old glass is in good shape, leave it in place.
The next step is to get rid of the old putty. If the putty is badly cracked, you can pry away large chunks quickly (Photo 1). Putty in good condition takes longer to remove. With a heat gun in one hand and a stiff putty knife in the other, heat the putty to soften it and gouge it out. Wear leather gloves to protect your hands from burns. Keep the heat gun moving to avoid concentrating heat in one spot. Otherwise the heat will crack the glass. If your heat gun doesn't have a heat shield attachment, protect the glass with a scrap of sheet metal. When the putty is removed, prime any bare wood inside the window frame. A shellac-based primer such as BIN is a good choice because it dries in minutes.
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Step 2: Replace the glass and putty the window
If you need new glass, measure the opening, subtract 1/8 in. from your measurements and have the new glass cut to size at a full-service hardware store. Take a shard of the old glass with you to match the thickness. Also buy a package of glazing points to hold the glass in place while the new compound hardens. Glazing compound is available in oil-based and latex/acrylic versions. The latex products, which usually come in a tube, have a longer life expectancy and you don't have to wait days before painting them as you do with oil-based putty. But they often begin to dry before you can tool them smooth. If neat, smooth results are important, choose an oil-based putty.
For installation of new glass, the directions on glazing compound may tell you to lay a light bead of compound inside the frame and then set the glass over it. That works well with soft latex compound. But if you're using stiffer oil-based compound, lay in a light bead of acrylic latex caulk instead. Set the glass onto the caulk, then wiggle and press down to firmly embed the glass. Then apply new putty as shown in Photo 3.
To complete the job, smooth out the new glazing compound (Photos 4 and 5). Oil-based putty is easier to work with when it's warm. To heat it, set the can in a bowl of hot water for a few minutes. Remember that oil-based putty remains soft for days, so be careful not to touch it after smoothing. You'll have to wait several days before you can prime and paint oil-based putty; check the label.
An alternative to glazing compound
An Alternative to Putty: Mitered Moldings
Applying a smooth, perfect bead of glazing compound is fussy, time-consuming work. So when good looks matter, consider wood moldings rather than putty to hold glass in place (1/4-in. quarter round works for most windows). Set the glass in place over a light bead of latex caulk (see Photo 2). There's no need for glazing points. To nail the moldings in place, you can carefully drive in tiny brads with a hammer or carefully shoot in brads with a pneumatic brad nailer. But the safest method is to use a brad pusher. A brad pusher is simply a metal tube with a sliding piston inside. Drop a brad in the tube, push hard on the handle, and the piston pushes the brad neatly into wood—with little danger of breaking the glass. Most hardware stores and home centers don't carry brad pushers, but you can find them at woodworker supply stores or online.