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.