How to Re-Putty a Sash Window Without It Cracking

You can see daylight where the putty has pulled away from the glass, and there are crumbs of it on the cill every time the window is opened. Somewhere behind that gap, rain is sitting in the rebate against bare timber. Re-puttying is one of the few sash window jobs a careful homeowner can genuinely do properly, but almost every failed putty job fails for the same two reasons, and both of them happen before the putty ever comes out of the tub.

Why putty cracks a year after it goes on

Linseed oil putty is whiting (ground chalk) bound with linseed oil. It does not dry the way a filler dries. It cures by oxidation — the oil reacts with air, skins over, and then hardens slowly inwards over months and years. That single fact explains nearly all putty failure.

Put fresh putty into a bare, unprimed rebate and the timber behaves like blotting paper. It draws the oil straight out of the putty on the timber side. The bead loses the binder it needs to cure properly, goes powdery, shrinks, and lets go of the glass. This is the first missed step, and it is the one that guarantees a crack within a year.

The second is paint. Putty that is never painted stays exposed to sun and rain, loses its remaining oil to UV, and crumbles. Putty that is painted too soon has an uncured, soft body sealed under a film. Putty that is painted without lapping the paint slightly onto the glass leaves an open seam at the glass line, which is exactly where water runs.

The third cause is less common but worth knowing. If the glass itself is rattling in the rebate because the sprigs are missing or corroded, the pane flexes every time the sash slides and every time a lorry goes past. Putty cannot hold glass still on its own. That is a mechanical fixing job, not a putty job, and loose glass is one of the quieter causes of a sash that rattles in the wind.

Choosing the right material

For an original London sash with single glass, linseed oil putty is the right answer and it is also what a conservation officer will expect to see. It stays slightly flexible for years, it takes paint, and when it eventually needs redoing in fifteen or twenty years it comes out without wrecking the rebate. Acrylic compounds have their place — a cracked pane in February, when linseed will simply sit there refusing to skin — but they are less forgiving on old absorbent timber and they do not weather in the same way. There is more on the trade-offs in our longer piece on linseed versus acrylic glazing putty.

One absolute: never bed a sealed double glazed unit in linseed oil putty. The oil attacks the perimeter edge seal, and a unit glazed that way can mist internally within months. Manufacturers of slim retrofit units are explicit about it — units go in on silicone or glazing tape, with a heritage-grade putty used only for the visible external bevel. If you are considering slim double glazed units in original sashes, that detail alone is worth understanding before anyone quotes you.

Getting the old putty out without breaking the glass

Old glass is worth more than the labour. Original drawn or cylinder glass is thinner than modern float, full of gentle distortion, and once broken it cannot be matched from a merchant — replacements come from salvage or from specialist heritage glass suppliers at real cost. Work as if the pane is irreplaceable, because it more or less is.

1. Take the sash out and lay it flat on a bench or trestles, glass upwards, supported all over. Half the broken panes in this job happen because someone worked on a sash that was still hanging and flexing.
2. Score along the paint line where paint bridges putty to glass, so you are not tearing at a skin that runs onto the pane.
3. Soften the putty before you cut it. An infrared softener is the heritage standard because it works below the temperature that shocks glass. A hot air gun on its lowest setting, kept moving and never held on one spot, will do it too. Uneven heat is what cracks glass: one hot patch expands while the rest of the pane does not.
4. Cut the bead out with a hacking knife, driven with light taps rather than levered. Work along the bead, not into the glass.
5. Pull the sprigs with pincers. Keep the good ones if they are still sound.
6. Scrape the rebate back to clean, sound timber. Vacuum it out. If the timber under the old putty is soft, dark, or takes a bradawl without resistance, stop — that is wet rot in the rebate and it needs cutting out and splicing before any glazing happens.

Priming the rebate — the step that decides everything

Once the rebate is clean and dry, prime it. An oil-based primer, worked well into the corners and the back of the rebate, then left to dry properly. On bare or partially weathered timber this is not optional: the manufacturers' own data sheets say to re-prime any section of rebate that has been missed or badly weathered before glazing.

The primer seals the timber so it cannot rob the oil out of the fresh putty. That is the whole mechanism. Skip it and everything else you do is wasted labour.

Do not glaze onto a damp rebate. If the timber has been rained on, wipe it out and give it a day. Putty over trapped moisture pushes the water into the timber it is meant to protect.

The glazing sequence

Work the putty in your hands first until it is warm and plastic — a couple of minutes of kneading. If it is sticky and oily, roll it on brown paper or newspaper to draw off the surplus. If it is crumbly and short, a few drops of raw linseed oil worked in will bring it back. Cold putty tears and drags; do not attempt this on a bench in an unheated garage in January.

1. Back bed. Run a continuous ribbon of putty around the rebate, thick enough to leave a bed of roughly 1-2mm under the glass all the way round. The glass must not touch bare timber anywhere.
2. Set the pane. Lower it in onto small setting blocks at the bottom, centre it in the opening, then press it home with steady pressure at the edges — never the middle. You want that 1-2mm bed squeezed out evenly.
3. Sprig it. Push glazing sprigs into the rebate against the glass, roughly every 200-300mm and at least two per side, sliding the tool along the glass face rather than tapping towards it. These, not the putty, are what hold the pane.
4. Front it. Press putty firmly into the angle between glass and rebate all the way round, more than you think you need.
5. Knife the bevel. Hold the putty knife at roughly 45 degrees and draw it in one continuous stroke along each side, thumb steadying the blade. The face of the bevel should finish just below the internal sight line — about 2mm shy of it — so that from inside the room you see timber and glass, not a line of putty.
6. Cut the corners. Run the knife out through the corner at 45 degrees each way to make a clean mitre. Sloppy corners are the tell of an amateur job and they are also where water collects.
7. Knife off the back bedding. Inside, trim the squeezed-out bed flush, sloping the cut away from the glass so any condensation runs off rather than sitting against timber.
8. Brush it. A soft, dry brush drawn lightly along the bead beds the putty to the glass and closes any pinholes. This step takes a minute and adds years.

Clean the surplus off the glass while it is fresh. Once it has skinned, you are scraping.

Painting: the timing that people get wrong

Leave the putty until it has formed a skin firm enough to take a brush without dragging. In practice that is usually 7 to 21 days depending on temperature and humidity — a warm dry spell will do it in a week, a damp London January can take three. Press it gently with a fingertip: firm, no print, no give.

Then get the first coat on. Manufacturers' guidance is a first coat once skinned, generally within 7 to 21 days, and the final coat within 28 days. Leaving it much longer defeats the point, because from the moment it is exposed the putty is losing oil.

When you paint, carry the paint 1 to 2mm onto the glass. That thin lap is the actual weather seal. It bridges the joint between paint film, putty and glass so water cannot track down behind the bead. A cut line that stops short of the glass looks tidier for a season and then lets the whole job down. The order of work for the rest of the sash — which face to do first, how to avoid gluing the sash shut — is covered in our guide to painting sash windows properly.

When re-puttying is not the right job

Be honest about what you are looking at before you buy a tub of putty.

• The rebate is soft. Putty into rotten timber is decoration over a problem. Cut it out and splice in new.
• The bead is sound but the paint has gone. You do not need to hack out good putty. Prepare and repaint; putty that is still adhering and flexible has years left.
• Every pane on every sash needs doing. Twelve panes over six sashes is a two-weekend job for a homeowner and half a day for a joiner with the sashes out. At that point the sums change.
• You want double glazing. Retrofitting sealed units into original sashes is a different craft with different rebate depths, different sealants and different failure modes — see our double glazing work for what is involved.

If you are working through a whole elevation, do the puttying as part of a proper overhaul rather than on its own: sashes out, cords checked, beads eased, putty done, everything primed and painted in one sequence. It is the same access, and access is most of the cost. Our seasonal maintenance checklist sets out what else is worth doing while the sashes are out.

And if the pane is cracked, the rebate is rotten, or the sash is on a second-floor bay over a pavement, that is the point to stop. Our sash window repair service covers every London borough, and a joiner will have the sash out, re-glazed and back in before a Saturday afternoon has gone.