A customer asked us a fair question last week. What is the lifespan of Sash Glaze, compared with the 50-plus years you get out of linseed oil putty?
It deserves a proper answer rather than a sales pitch, including the parts that do not flatter us.
The short version
Linseed putty can absolutely reach 50 years. We have pulled 90 year old putty out of windows in Hobart that was still doing its job. But the putty is not what got it there. The paint over the top is what got it there, and the same is true of every glazing compound on the market, ours included.
How linseed putty actually behaves
Linseed oil putty is whiting, which is finely ground chalk, bound with linseed oil. It does not dry the way paint does. It cures by oxidation, meaning the oil reacts with oxygen and slowly hardens over months and then years.
The problem is that the reaction never really stops. The putty keeps oxidising and keeps losing oil for its entire life. Early on this is a good thing, because it is what turns a soft bead into a firm one. Twenty or thirty years down the track it is the reason the bead has gone hard, chalky and slightly smaller than the rabbet it was pressed into.
It also loses oil downwards, into the timber. This is why a properly done job primes the rabbet first. Bare, thirsty timber will pull the oil straight out of the back of the bead and you end up with a putty line that fails from behind, where you cannot see it, years before the face of it looks tired.
None of this is a design flaw. It is just what the material is. It has been doing the job since the 1600s and it will keep doing the job.
The paint line is the thing
The paint has to run up off the putty and onto the glass by a couple of millimetres. That small overlap is the seal. It stops water tracking down behind the bead and it keeps ultraviolet light off the putty surface.
While that overlap is intact, the putty sits inside a protected envelope. Oxidation slows right down, and neither UV nor standing water gets at it.
The day that paint line cracks or peels back off the glass, the clock starts. Air gets under the film, UV starts hitting the putty directly, moisture gets in, the oils leave faster, and the bead begins to shrink and lift. From there it is usually a handful of seasons before it is crumbling out in pieces.
So when someone quotes you 50 years for linseed putty, what they are really describing is a window that somebody kept painting. Under a well maintained coat in a sheltered aspect, 30 to 50 years is realistic. On an exposed western elevation with dark paint and no touch-ups, 10 to 15 is closer to the truth, and we see plenty of that.
Where hybrid polymer fits in
Sash Glaze is a hybrid polymer. The chemistry is a silane-terminated polymer, sometimes called an MS polymer, which was developed in Japan in the late 1970s and has been used in construction sealants ever since.
It cures a completely different way. It reacts with moisture in the air, skins over in about twenty minutes and is fully cured within a day. Once it is cured, that is it. There is no ongoing oil loss and no continuing oxidation. Nothing has to keep leaving the bead for it to go on working.
The practical result is a bead that stays flexible instead of getting progressively harder. Published figures for this class of material sit around 150% elongation at break and a Shore A hardness of about 60, across a service range of roughly minus 40 to plus 80 degrees. Timber moves seasonally and glass expands and contracts on a hot day. A flexible bead moves with it. A brittle one cracks away from the glass.
It also bonds to slightly damp timber, which putty will not do, and it does not bleed oil into the substrate or stain the surrounding paintwork.
And it needs paint too
A lot of marketing gets dishonest here, so we will be straight about it. Hybrid polymer glazing is not a set and forget product. It gets painted, exactly like putty does, with the same overlap onto the glass. The paint is still the weathering surface, it still takes the UV, and it still needs looking at every few years and touching up where it has let go.
Paint is the answer to longevity for both materials. Anyone selling you a glazing compound on the basis that it removes the need to maintain the paint is telling you a story.
What differs is what happens when the paint does fail, because it eventually will on some elevation of some building. With putty, once it is exposed, the failure mechanism is already loaded and it accelerates. With a cured hybrid polymer, the bead itself is UV stable and still elastic, so it keeps sealing while you get around to sorting out the paint. You end up repainting rather than reglazing.
The bit where we admit the comparison is unfair
Linseed putty has four centuries of buildings behind it. You can walk up to a Georgian window and read the result for yourself.
Hybrid polymers do not have that, and nobody can hand you a 50 year old bead of the stuff, because the chemistry has not existed for 50 years in this application. Any manufacturer claiming otherwise is extrapolating, and you should say so out loud when they do it.
Manufacturers of this class of glazing compound typically publish a minimum 10 year service life with a matching guarantee. That is a conservative, defensible number, and it is what we would put in writing.
What we can add to it is our own field record. Sealasash has been using this material since the business started in Hobart in 2011, across more than 20,000 windows, across Tasmanian coastal exposure, Sydney humidity and Melbourne's temperature swings. In 15 years we have not had a Sash Glaze bead fail, and we have never had a callback for a cracked or lifted bead. That is longer than the guarantee period and it covers a lot of weather.
It is not a laboratory result and we would not pretend otherwise. It is just what we have seen on our own jobs, and it is the reason we keep specifying it.
What this means if you own an older home
Whichever material is in your rabbets, do this once a year and it is worth more than any product choice you will make.
Walk around the outside with a coffee and look at the paint line where the glazing meets the glass. You are looking for hairline cracks, a paint edge that has pulled back off the glass, or a chalky dull patch. Any of those, get a brush on it before winter. That single habit will add ten to fifteen years to a putty bead and it costs an afternoon.
If your putty is already crumbling out in sections, that is a reglaze or a repair rather than a paint job. Sash Glaze can be run over existing putty that is cracked but still sound and adhered, which seals the water out and buys a lot of time without pulling the glass. Where the putty has gone entirely, it goes back in as a full replacement bead and it can be painted the same day rather than waiting weeks for putty to skin.
That last point matters more than it sounds. On an occupied building, or a job with a two week window before term starts, the difference between painting today and painting in a month is the difference between the job happening and the job not happening.
Bottom line
Putty is a good material. It has earned its reputation and there is nothing wrong with using it.
But the 50 year figure belongs to the paint, not the putty, and once you understand that the comparison looks different. We use hybrid polymer because it removes the built-in decay mechanism, holds its flexibility, goes back to paint the same day, and because in 15 years and 20,000 windows we have not had one let go.
Keep painting either way.
Get Sash Glaze
Sash Glaze Instant Adhesive 290ml is the hybrid polymer glazing compound described in this article. One 290ml tube does roughly 8 to 10 metres of bead. It goes over cracked but sound putty as a repair, or straight into a stripped rabbet as a full replacement bead, and it takes paint the same day.
Buy Sash Glaze from $21.99, with carton pricing for trade quantities.
Sealasash has been restoring and upgrading heritage timber windows since 2011, with teams in Sydney, Melbourne, Hobart, Launceston and Perth.