How do I re-putty a timber window?

Putty fails when it dries out and shrinks away from the glass, and once it cracks, water gets into the rebate and rots the timber behind it. Re-puttying is the maintenance job that keeps a timber window alive for another twenty years.

What you'll need

• Linseed oil putty
• A hacking knife or old chisel
• A good putty knife
• Primer or boiled linseed oil
• A heat gun on a low setting
• Glazing sprigs, if any are missing

Step by step

1. Assess how much has failed.Press along the fillet. Sound putty is hard and firmly attached; failed putty is loose, cracked or has a visible gap against the glass. Only remove what has failed.
2. Rake out carefully.A hacking knife worked along the rebate, or a sharp old chisel used at a shallow angle. Keep the blade moving along rather than levering outward.
3. Use heat sparingly.A heat gun on low softens stubborn putty, but glass cracks from uneven heating. Keep the gun moving, keep it off the glass, and shield adjacent panes.
4. Check the sprigs.With the putty out, the small headless pins holding the glass are visible. Replace any that are missing before the new putty goes on.
5. Clean and prime the rebate.Brush out the dust and prime the bare timber, or give it a wash of boiled linseed oil. This is the step that decides whether the putty lasts two years or twenty.
6. Knead the putty.Work a handful until it is warm, smooth and just tacky. If it is oily, roll it on newspaper; if it is crumbly, work a little linseed oil in.
7. Press it into the rebate.Roll a sausage and thumb it firmly into the angle between the glass and the rebate, pushing hard enough to expel air and bond it to both surfaces.
8. Strike it off in one pass.Hold the putty knife at a constant angle and draw it along in a single confident movement. Hesitation and repeated passes are what produce a wavy line.
9. Aim for the right height.The finished fillet should sit just below the sight line of the glazing bar seen from inside, so the putty is invisible from the room.
10. Mitre the corners.A clean diagonal cut at each corner with the corner of the knife. It is the detail that separates a tidy job from an obvious one.
11. Wipe the glass while it is fresh.Any putty smeared on the glass comes off easily now, and with difficulty in a week.
12. Wait, then paint.Two to four weeks for a skin to form, depending on weather. Then prime and paint, taking the paint 2mm onto the glass.

Tips

• Dip the putty knife in water occasionally. It stops the putty dragging and gives a noticeably smoother line.
• If the fillet keeps tearing, the putty is too dry. A few drops of linseed oil worked in makes it behave.
• Do this in mild, dry weather. Putty applied in cold, damp conditions does not skin properly and is far harder to work.

Common mistakes

• Painting the putty before it has skinned — Paint over uncured putty seals the surface, stops the linseed oil oxidising properly, and the film wrinkles and cracks. The putty underneath stays soft for years.
• Leaving the putty unpainted — The other extreme. Unpainted linseed putty loses its oil to the weather, hardens, shrinks and cracks away from the glass within a couple of seasons. It needs paint, but not too soon.

If it doesn't work

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