What happens when the putty on your window glass wears out and needs replacing? Note that this is completely separate from (and, thankfully, cheaper than) having to replace the window itself. The assumption here is that the window glass itself is fine. Actually, this same method works when you do need to replace a window, but there is more to that.
The putty is what creates a seal against the elements between the window glass and the frame. Without it, or when it develops holes and gaps, you lose effective insulation and make your home less energy-efficient, and also let in cold drafts.
Luckily, replacing the putty is fairly simple and requires only a few tools, including: putty, a putty knife, a heat gun or hair dryer, and glazier’s points and a glazing tool.
First: remove the old putty. Use a putty knife, and for really stubborn spots a heat gun or hair dryer to soften it. Work on small parts at a time, breaking the putty into chunks with the putty knife and removing it. Go over the window to remove all of old putty; you want a clean surface to apply the new.
Next: remove the window glass. Press it carefully from the opposite side from where you removed the putty from to lift it out of the frame. Set it aside somewhere that it won’t break.
Third: go over the frame again after removing the glass and remove any remaining putty that you couldn’t get to because the glass was in the way.
Fourth: spread a light coat of linseed oil on the frame to keep the putty from drying too quickly.
Fifth: apply a coat of putty to the frame in the slot where the window pane came from, to a thickness of about 1/16 inch.
Sixth: press the glass into the putty. Using even pressure, push it down firmly.
Seventh: insert a glazier’s point every four or five inches all around the frame. To do this, lay the point down flat with the point towards the frame, and drive them into the frame with the glazing tool. This holds the glass in place.
Eighth: apply more putty (again not very thickly) atop the glazier’s points. Smooth the putting with the glazing tool, not allowing it to spread more than about ¼ inch over the glass. From the other side of the window, the putty should be invisible; if you can see it, it’s too thick.
— Cornish Lime (@Cornish_Lime) April 6, 2017
Finally, paint the putty to match the décor and seal it against the elements. You can find some more about replacing old putty here.