How to Fix a Cracked Window Glazing: Step-by-Step Guide

Old houses are full of single-pane wood windows with glazing that has gone hard, cracked, and crumbling. Beyond looking shabby, failed glazing lets cold air whistle in and moisture work its way behind the glass — eventually rotting the wood sash.

Old houses are full of single-pane wood windows with glazing that has gone hard, cracked, and crumbling. Beyond looking shabby, failed glazing lets cold air whistle in and moisture work its way behind the glass — eventually rotting the wood sash. Reglazing is a satisfying repair that costs under twenty dollars per window and dramatically improves energy efficiency. Here is exactly how to do it.

Tools and Materials You Will Need

• Glazing compound (oil-based recommended)
• Putty knife (stiff, 1-inch blade)
• Heat gun or propane torch
• Linseed oil or boiled linseed oil
• Glazier’s points
• Paintbrush
• Exterior primer and paint
• Safety glasses
• Drop cloth

Step 1: Assess the Window

Work on one window at a time. Examine all four sides of each glass pane from the exterior. Look for glazing that is cracked into pieces, pulling away from the glass, or missing entirely. Press the old putty lightly — it should feel firm and slightly flexible. If it crumbles or is completely hard like stone, it needs to go.

Step 2: Remove the Old Glazing

This is the most labor-intensive part. Use a stiff putty knife to pry away loose chunks. For hardened compound that will not budge, use a heat gun on low setting, working in small sections and scraping as the compound softens. Move the heat gun constantly to avoid cracking the glass.

Do not force the putty knife aggressively against the glass. Work at a low angle and let the tool follow the wood frame rather than prying against the pane.

Once the old putty is out, you will see small metal tabs called glazier’s points holding the glass in place. Leave these alone if they are intact. If any are missing or loose, press new ones in flush with the wood using the tip of your putty knife.

Step 3: Prepare the Bare Wood

With the old compound removed, the exposed wood rabbet (the groove where the glass rests) needs to be primed so it does not absorb all the oil from your new glazing compound. Brush on a thin coat of raw linseed oil or diluted exterior primer and let it soak in for 30 minutes. Wipe off any excess. This step is critical — skipping it causes new glazing to dry out too fast and crack within a few years.

Step 4: Apply New Glazing Compound

DAP 33 Window Glazing Compound (Amazon, tag=fixupfirst-20) is the standard choice and widely available. Knead a golf-ball-sized amount in your hands until it is pliable and smooth. Roll it into a rope about the diameter of a pencil.

Press the rope firmly into the corner where the glass meets the wood, working one side at a time. Then drag your putty knife along the compound at a consistent 45-degree angle, pressing firmly to push it into the corner and creating a clean beveled surface. The bevel should run from the wood frame down to the glass without any gaps or air pockets.

Smooth tool marks by running a wet fingertip lightly over the surface. Corners can be trimmed clean with the edge of the putty knife.

A flexible glazing knife set (Amazon, tag=fixupfirst-20) with multiple blade profiles makes it much easier to achieve a clean angle on all four sides of the pane.

Step 5: Let It Cure Before Painting

Allow the glazing compound to cure for at least 7 to 14 days before painting. The surface will feel dry to the touch within hours but is not ready for paint. Painting too soon traps moisture and leads to early cracking.

Once cured, apply one coat of exterior primer followed by a finish coat of exterior paint. The paint should lap slightly onto the glass — about 1/16 of an inch — to create a complete weather seal between the compound and the glass surface.

Step 6: Inspect Your Work

Run your finger along every edge of the new glazing. There should be no gaps, no lifting edges, and no areas where you can feel air movement. Any thin or short sections should be filled with a small additional amount of compound, smoothed in with your fingertip, and allowed to cure again before painting.

Reglazing a window properly takes about 45 minutes per sash. Done right, new glazing will last 20 years or more.