Preparation
Inspect your window before starting the removal process. Notice where the glazing has, essentially, started chipping away by itself; plan to start removal here–usually where the putty meets the wood, not where it meets the glass. Maintain a steady work surface if the window is not in place.
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Removal Method
Use a putty knife, paint scraper, utility knife, chisel or any combination of tools that works well for you, to remove the putty, concentrating first on areas where the putty is deteriorated and/or missing. Carefully scrape, pry and work at removing the glazing, taking care not to break the glass if it is still present in the window. Work a small section at a time until you come to a push point, sometimes called a glazing point. Spaced every 6 to 8 inches generally, these points hold in the glass.
Finishing Removal
Avoid loosening the glazing points as you work, and don’t dig your knife or tool into the wood frame. Use a heat gun or similar device to carefully heat the window glazing slightly to help soften and loosen it for removal if needed. Sand with sandpaper to finish the glazing removal, and provide a fresh new surface for your new window installation.
References:
Easy2DIY: Replacing Glass Tutorial
Old House Web: Repairing Wood Windows
DIY Network: How to Repair and Repaint Old Window Sashes
Dave Vandervorts Home Tips: Replacing Brittle Window Glazing Compound
DIY Online: Windows–Replacing Glass
Resources:
DIY ThisNThat: How to Glaze Old Windows–Step by Step Guide
Here and There: Repairing Old Wood Windows – Part 1: Repairing and Removing Glazing and Paint