Have the new glass cut so it measures 1/8 inch shorter than the exact groove dimensions in both the length and width. This creates a 1/16-inch gap on each side between the edges of the pane and the rabbet groove. The gap provides room for the glass to expand when the weather changes.
In addition to the replacement glass pane, you'll need: latex glazing putty, pliers, heat gun, flexible putty knife, 1- or 2-inch stiff steel putty knife, and a flathead screwdriver.
Use the pliers to remove all of the glass shards.
When you work with broken glass, wear safety goggles as well as gloves; small chips of glass can cause permanent eye damage.
Use the heat gun to warm the old putty; and then scrape it away with a putty knife.
If the putty doesn’t lift off easily, apply more heat and try again. Be patient —the putty around really old windows is as hard as concrete, but it will soften.
Don’t chisel out the old putty — you might wreck the window. The putty will soften with enough heat and time.
Use a putty knife or the tip of a screwdriver to remove the spring clips.
The clips will be buried under the old putty. Hold on to these clips so that you can use them again.
Clean and inspect the rabbet groove.
Ensure that no glazing putty, glass shards, or spring clips remain.
Apply a bead of putty on the glass side of the rabbet groove.
Squeeze out a 1⁄16-inch bead of putty between the edge of the glass and the window frame.
Gently press down the glass at the edges to bed the glass into the putty.
Allow the putty bed to spread out and form a moisture seal on the inside of the window between the glass and sash.
Position the new pane in the rabbets.
Shift the pane until you have a 1/16-inch gap between the pane and sash on all four sides.
Place at least two spring clips in each section of the window sash surrounding the new glass.
Space the spring clips (the ones that you removed earlier) evenly around the perimeter,
Push each spring clip into the sash.
You can use the tip of a screwdriver to help push in the spring clip.
Form a 1/2-inch-thick rope of putty and press the length along all four sides of the glass.
Roll a glob of putty between your bare hands to form the rope.
Smooth the glazing putty and scrape away the excess.
Hold the putty knife at a 45-degree angle, press, and smooth the glazing putty against the glass and sash.
After the putty dries completely, repaint the putty and repaired area.
Don’t mask off the glass before repainting. The paint will help form a moisture seal between the glass pane and the sash. So, allow the paint to overlap about 1/8 inch onto the glass.
