Why glazing putty cracks and falls out — seven failure patterns
Putty that crumbles, lifts off in lengths, stays greasy for months or splits away from the glass is telling you exactly what went wrong. How to read each pattern, and what to do about it.
Putty rarely fails for mysterious reasons. It fails in a small number of recognisable patterns, each with a specific cause, and the pattern is usually visible from three feet away. Learning to read them saves you from the two standard mistakes: replacing putty that only needed painting, and painting over putty that needed replacing.
Here are the seven patterns, roughly in order of how often they turn up on British windows.
1. Fine surface crazing, putty still hard and firmly attached
What you see. A network of hairline cracks across the face of the fillet, like old glaze on a teapot. Press it and it is rock solid. It has not moved away from the glass or the timber.
What it means. Nothing much. This is an unprotected surface ageing. Linseed putty cures by taking up oxygen, and it keeps taking up oxygen for its whole life; the outer half-millimetre eventually goes brittle and crazes. If the paint film above it is thin or has worn back, this is what you get.
What to do. Do not hack it out. Brush the loose dust off, prime the crazing, and paint. A sound fillet with a crazed face has years left in it, and every fillet you replace unnecessarily is an hour of work and a small risk to the glass.
Get it wrong how? By treating this as failure and stripping a perfectly good window.
2. A dark line opening between putty and glass
What you see. A thin dark line running along the top of the fillet where it meets the pane, sometimes with the paint visibly split along it. Often worst on the bottom edge and on south and west elevations.
What it means. The paint seal has broken. Putty and glass are two materials with different movement, and the joint between them is bridged by paint alone. Once that bridge splits, water runs into the joint every time it rains, sits there, and works its way behind the fillet and into the rebate.
What to do. Catch it early and it is a decorating job: rake the joint out with a knife point, brush it clean and dry, re-face that run of putty if it has opened up, and repaint with the paint carried 1–2 mm onto the glass. That overlap is not optional. It is the seal.
Left for a few years, the same defect turns into pattern 7 — wet rot in the rebate.
3. The fillet lifts off in whole lengths, undamaged
What you see. You put a knife under the putty and a length of it comes away in one piece, perfectly formed, with a dry chalky face where it met the timber. Sometimes it falls off on its own after a frost.
What it means. Oil starvation, and almost certainly an unprimed rebate. Bare softwood is intensely absorbent. Fresh putty pressed against it loses its linseed oil into the timber within hours, and putty without oil cannot cure — it just sits there as compressed chalk until something knocks it loose.
You can confirm it instantly: look at the back of the piece that came away. Cured putty is a uniform waxy colour through its thickness. Starved putty has a pale, dusty, chalk-like zone where it touched the wood.
What to do. Remove all of it — every bit of that fillet has the same problem. Prime the exposed rebate with a solvent-borne wood primer, let it dry, and re-putty. This is not a repairable defect; it is a redo.
Also causes it: puttying onto timber that was primed years ago and has since weathered back, and puttying onto dense hardwood with an ordinary timber putty, which is the mirror-image problem (see pattern 5).
4. The putty crumbles to powder under a fingernail
What you see. The fillet has no coherence left at all. Bits fall out when you touch it. What remains is pale and dusty rather than waxy.
What it means. Long-term ultraviolet exposure with no paint over it. UV breaks down the cured oil at the surface, and with nothing to stop it the degradation works progressively deeper. It is the end state of “we’ll paint it next year”.
The speed surprises people. A single hot summer on a south-facing elevation is enough to craze bare putty right through; two or three years of full exposure will powder it.
What to do. Full removal and replacement of that fillet, and then paint it inside 28 days — which is what the manufacturers’ data sheets ask for, and the reason they ask.
5. Putty that never sets — soft and greasy months later
What you see. Weeks or months after glazing, the fillet still takes a thumbprint. It may look faintly oily. It certainly cannot be painted.
What it means. One of three things.
- Wrong putty for the substrate. Ordinary multi-purpose putty part-relies on an absorbent frame drawing a little oil out of it to begin curing at the back. On primed steel — or on a very dense hardwood — nothing is drawn out, and the putty stays soft against the frame indefinitely. Steel casements need metal casement putty, which is formulated to set without that help.
- It was applied too cold. The cure is an oxidation reaction and it effectively stalls below about 5 °C. Putty applied in a cold snap can sit unchanged for weeks and then start curing when spring arrives.
- Over-oiled putty. Usually self-inflicted, by adding linseed oil to soften a stiff tub and going too far. Roll putty out on brown paper to draw excess oil off instead.
What to do. If it is a cold-weather stall, wait for warmth: it will usually come good. If it is the wrong material or over-oiled, take it out and redo it with the right putty. Nothing you paint on will rescue an uncured fillet.
6. The glass rattles, but the putty looks fine
What you see. Tap the frame and the pane moves audibly. The external fillet may look perfectly sound.
What it means. The fixing has failed, not the face. Either the sprigs were never fitted, or they have corroded away, or the back bedding has perished and the glass is now sitting on nothing.
This is worth taking seriously. A rattling pane is a pane that flexes with every gust, and glass that flexes in a hard frame is glass that eventually cracks — usually with a curved crack running from an edge.
What to do. This one needs a full re-glaze, not a re-face. The pane comes out, the rebate gets cleaned and primed, new sprigs go in every 200–250 mm, and the glass is bedded on fresh putty. Fixing only the outside face leaves the actual defect in place.
7. Soft, dark timber under the fillet
What you see. The putty is failing and the timber it sits on is discoloured, soft to a bradawl, or visibly crumbling — classically in the bottom corners of the rebate, where water collects.
What it means. Wet rot, which is where patterns 2 and 3 end up if nobody intervenes. Water has been getting behind the fillet for years.
What to do. The timber comes first; putty over rot fails again with the rot underneath it. Small areas cut out and rebuilt with a two-part epoxy joinery repair system are a legitimate, long-lasting fix. A bottom rail that has gone along its length needs a spliced timber repair, which is joinery rather than decorating. Only once the rebate is sound and dry should it be primed and re-puttied.
The quick diagnostic table
| What you see | Cause | Fix |
|---|---|---|
| Hairline crazing, putty hard | Normal surface ageing | Clean, prime, repaint |
| Dark line at the glass | Paint seal has split | Rake out, re-face if open, repaint over the glass edge |
| Lifts off in lengths, chalky back | Oil starvation — unprimed rebate | Remove, prime, re-putty |
| Crumbles to powder | UV, left unpainted | Remove, re-putty, paint within 28 days |
| Still soft after months | Wrong putty, too cold, or over-oiled | Wait if cold; otherwise remove and redo |
| Glass rattles | Sprigs or back bedding gone | Full re-glaze |
| Timber soft and dark | Wet rot | Repair timber first, then re-putty |
The three causes behind almost everything above
Strip the patterns back and nearly every putty failure in Britain traces to one of three things.
An unprimed or badly primed rebate. The most common by a distance, and the cheapest to avoid: one coat of primer, ten minutes, before the putty goes anywhere near the frame.
Putty left unpainted. The paint is not decoration. It is the weatherproofing that keeps UV off the oil and water out of the joint. Skin forms in 2–4 days; the first coat wants to be on within 7–21 days and the final coat within 28.
Water sitting somewhere it shouldn’t. Behind a discontinuous bed of back putty, in a split paint joint, in the end grain of an unprimed corner. Putty is very good at keeping water out and completely defenceless once water is behind it.
Get those three right and the failure patterns on this page mostly stop happening. For the full sequence, see how to re-putty a window; for the painting schedule in detail, see painting glazing putty.
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