How to Glaze Wooden Windows with Putty
Glazing putty is what keeps glass in place and water out of a timber window frame. It does a lot of work for something most people never think about until it fails. When it does fail - cracking, crumbling, pulling away from the glass - you get draughts, moisture getting in, window frames rot and eventually loose panes. Replacing it is a straightforward job if you take your time. Here is how you replace glazing putty in single glazed wooden windows.
How do you know putty needs replacing?
It is usually obvious. Cracks running through the putty line, chunks missing where it has crumbled away, gaps between the putty and the glass, draughts you can feel around the window, condensation creeping in at the edges, or glass that rattles when the wind picks up. Any of those means the glazing has gone.
If the putty is cracked but still bonded and the window is not letting in water, you might get away with running a fresh line over the top. Most of the time though, failed putty needs to come out completely. Patching over the top of something that is already breaking down will not last.
Step 1: Prep - Strip Out and Prepare the Rebate
If you are replacing old putty, it all needs to come out. Use a putty knife or hacking knife and work carefully along the rebate. Take your time with this. Forcing it risks cracking the glass, especially on older single-glazed windows where the panes are thinner and more brittle.
Once the old putty is out, have a look at the timber rebate. Brush out all the dust and debris. If the wood is soft, spongy, or showing signs of decay, apply Wood Hardener to consolidate it. Putty needs a firm, stable rebate to bond to - if the timber is soft, it will not hold.
Apply Wood Hardener with the brush in the lid, working it into the grain and into the corners of the rebate. Let it dry fully before you put any putty in.
Even on sound timber, bare wood should be sealed before puttying. If you do not, the timber draws oil out of the putty too quickly and it dries out and cracks sooner than it should. A coat of wood primer or Wood Hardener will sort this.
Step 2: Bed the Glass
Before you start, work the putty in your hands until it is soft and pliable. Cold putty is stiff and fights you. Knead it until it reaches a smooth, consistent texture that is easy to shape.
Run a thin bed of putty along the rebate - roughly 3mm thick, in a continuous line. This is the cushion the glass sits on. It forms the seal between the glass and the frame on the inside face.
Set the glass onto the bed and press it firmly and evenly around the edges. You should see a small amount of putty squeeze out on the inside - that tells you the glass is making full contact. If nothing squeezes out, the bed is too thin or the glass is not seated properly.
Tap in glazing sprigs or pins at regular intervals around the frame to hold the pane in place. Be gentle - you are pinning the glass, not nailing it. The sprigs hold the pane while you apply the face putty.
Step 3: Face Putty
This is the visible putty line on the outside of the window. Roll the putty into a long sausage shape and press it firmly into the angle where the glass meets the frame. Work along each edge of the pane. Now draw your putty knife along the putty in one smooth stroke to cut it to a neat, angled profile. Begin by resting the blade against the glass on one side and the edge of the rebate on the other - the knife creates the angle for you. One continuous stroke per edge gives you the cleanest line. If you go back and forth you will end up with a messy finish. Trim off any excess from both inside and outside, and clean any marks off the glass with a dry cloth. The bead of putty should be no deeper than 5mm at its deepest, which is usually in the corners.
Painting - Do Not Skip
Unpainted putty dries out, cracks, and fails early. Painting is not optional - it is what protects the putty and makes the seal last. Let the putty form a skin before you paint. That usually means leaving it for seven to fourteen days depending on conditions. Check the product instructions for the exact time. When you paint, let the paint overlap onto the glass by 1-2mm. That small overlap creates a seal between the putty and the pane and stops moisture from working its way behind the putty line. It is a small detail that makes a big difference to how long the repair lasts. A properly puttied and painted window should be good for years before it needs attention again.