Linseed putty vs modern alternatives: which should you actually use?
MS-polymer sealants, butyl tape, silicone and acrylic compounds all promise to replace linseed oil putty. An honest comparison of cost, working life, appearance and — the decisive factor — what glass you are fitting.
Every few years someone announces the end of linseed oil putty. It is slow, it needs painting, it cannot be rushed, and it will not tolerate modern glass. All true. And yet it is still the default on single-glazed timber windows in Britain, for reasons that survive the comparison.
Here is the honest version, with the cases where the modern materials genuinely win.
The candidates
Linseed oil putty. Whiting and linseed oil. Cures by oxidation over weeks to months. Skins in 2–4 days; painted at 7–21 days, final coat within 28. A kilogram covers 3–5 metres of fronting and costs less than a takeaway. Made in the UK to the recipe of BS 544:1969 — a standard now withdrawn but still quoted on data sheets.
Metal casement putty. The same idea with a different oil blend, for non-absorbent frames: primed steel, dense hardwood, stone surrounds. See glazing putty for metal windows.
MS-polymer putty replacements. Modified-silane sealants gunned in and tooled to a bevel. The reference product in the UK is Repair Care’s Dry Seal MP. Cures in hours, stays permanently elastic, over-paintable in about two hours, and tested as compatible with laminated glass, sealed double-glazed units and slim heritage units. A 290 ml cartridge costs many times what the equivalent length of putty does.
Butyl compounds and glazing tapes. Non-setting. Used to bed sealed units and in bead-glazed and factory systems. Never harden, so they accommodate movement indefinitely; not a visible finish.
Silicone. A proper bedding material for sealed units, and a poor putty substitute: it cannot be knifed to a sharp arris and paint will not adhere to it.
Acrylic (water-based) glazing compounds. Skin and paint fast, shrink as the water leaves, shorter outdoor life.
The comparison that actually decides it
Most comparisons start with cost or cure speed. They should start with the glass, because that question is binary and it eliminates most of the options before preference gets a say.
| Single glass | Laminated | Sealed unit (incl. slim/heritage) | |
|---|---|---|---|
| Linseed oil putty | ✅ the default | ❌ attacks the interlayer | ❌ destroys the edge seal |
| Metal casement putty | ✅ on steel/hardwood | ❌ | ❌ |
| MS-polymer (e.g. Dry Seal MP) | ✅ | ✅ | ✅ |
| Butyl / tape | — bedding only | ✅ | ✅ |
| Silicone | ⚠️ can’t be painted | ✅ | ✅ |
| Acrylic compound | ⚠️ short life | ❌ | ❌ |
If your window holds anything other than a single pane of ordinary glass, linseed putty is out — not as a matter of taste, but because the oil will wreck the glass unit. That is the whole of the argument in most modern cases.
Where linseed putty still wins
Cost, on a big job. A terrace of sash windows is hundreds of metres of fronting. At 3–5 metres per kilogram, putty is the cheapest material in the whole operation. Doing the same work in cartridge sealant costs roughly an order of magnitude more.
Repairability. This is the underrated one. A putty fillet can be cut back in sections and re-faced, over and over, for as long as the joinery lasts. An MS-polymer fillet cannot be re-worked once it has skinned: you cut it out and start again. On a building expected to last another century, “endlessly patchable” is a real technical advantage, not nostalgia.
Appearance and acceptability on historic work. A knifed linseed fillet is what a conservation officer expects to see on a listed building, and what the window was designed around. Modern sealants tooled well look close; they do not look identical, and on a listed façade “close” is sometimes not the standard being applied.
Compatibility with linseed oil paint systems. Same oil chemistry, matched movement, and a maintenance cycle that has demonstrably worked for two hundred years.
No shelf-life politics. A tub of putty is a tub of putty. Kneaded back to life, a stiff tub still works. A part-used cartridge of MS polymer is usually scrap by the next job.
Where the modern materials win
Speed, and therefore weather risk. Putty needs 7–28 days before it can be painted, in weather it does not control. Dry Seal MP is over-paintable in about two hours. On a scaffolded job with a fixed window of dry weather — or on a commercial contract with a completion date — that is decisive, and no amount of tradition changes it.
Movement. Putty accommodates only a few per cent of joint movement; the data sheets say so plainly. Permanently elastic sealants accommodate far more. On a large pane, an exposed elevation or a frame that moves seasonally, that matters.
Modern glass. As above: laminated, acoustic, self-cleaning, sealed and slim units are all fine with a tested MS-polymer product and all incompatible with linseed oil.
Very exposed or awkward sites. Coastal, high-rise, north-facing, or anywhere access is expensive enough that you do not want to come back in three weeks to paint.
Consistency in unskilled hands. Knifing a good putty bevel is a skill. Gunning and tooling a sealant is easier to do acceptably on the first attempt.
What about “putty that dries faster”?
Marketing occasionally offers a fast-curing linseed putty. Treat the claim carefully. The cure is an oxidation reaction; the ways to speed it up are to add metallic driers, to thin the section, or to raise the temperature. Driers accelerate the surface at the cost of long-term flexibility — the fillet goes brittle sooner, which is precisely the failure mode you were trying to avoid.
If you genuinely need speed, use a material designed to be fast rather than a traditional material pushed to be fast.
Silicone: why it keeps happening, and why not to
Silicone appears on period windows constantly, because it is cheap, it is in every van, and it appears to solve the problem for an afternoon.
Two reasons it does not:
- Paint will not stick to it. So the fillet stays a shiny grey-white line against the paintwork, permanently, and no repaint will hide it.
- It cannot be knifed to a crisp arris. A tooled silicone joint has a soft, rounded profile that reads wrong at the sight line.
Add a third for the person who comes after you: removing cured silicone from a timber rebate is miserable, and silicone residue contaminates the surface so that neither paint nor putty will bond properly afterwards. A silicone repair therefore makes the eventual proper repair harder and more expensive.
Silicone is a good material used correctly — bedding a sealed unit, sealing a frame to masonry. It just is not putty.
A decision tree
- Is the glass a sealed unit, laminated or acoustic? → Not linseed putty. Use the unit maker’s specified sealant, tape, or a tested MS-polymer product.
- Is the frame bead-glazed? → Not putty. Bed on the sealant the system specifies.
- Is it single glass in a primed timber frame? → Linseed oil putty, unless speed or exposure genuinely rules it out.
- Is it single glass in steel or dense hardwood? → Metal casement putty.
- Is the building listed or in a conservation area? → Strong presumption toward like-for-like putty; ask the conservation officer before substituting anything.
- Do you need it painted the same day? → MS-polymer, and accept the cost.
The summary nobody wants to hear
There is no modern material that is better than linseed oil putty at everything, and no traditional argument that makes putty right for a sealed unit. The two live in different parts of the problem. Choose on the glass first, the building second, and the schedule third — and the answer usually picks itself.
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