We reglaze a lot of windows, so we get asked the same question all the time: is a hybrid polymer really better than the linseed oil putty that has held glass in for a hundred years? Rather than argue about it, we glazed the same double hung window two ways. One side with traditional linseed putty, the other with our hybrid polymer sash glaze. Same glass, same timber, same shed, same week. Here is what we saw.
The test
We took an old double hung sash out of a workshop window and split it down the middle. The bottom two panes went back in with linseed oil putty bedded and knifed off the old way. The top two went back with hybrid polymer, run in as a bead and tooled smooth. Both sides sat in the same conditions through the same cold snap, so nothing was stacked in favour of one or the other.
Before any of that we scraped out the old putty. If you are doing this on a heritage window, the old putty and the old paint around it can contain lead, so wear gloves, keep the dust down, and wash your hands before you eat or touch your face. That advice applies whichever product you choose to reglaze with.
If you want the full method for pulling glass and rebedding it, we wrote it up separately in how to reglaze a window without putty. This article is about the comparison, not the step by step.
Cure time compared
This is where the two materials part company fastest.
Linseed oil putty skins over in a few days, but skinning is not curing. Underneath that skin it stays soft for weeks, and in cold or damp weather it can sit soft for a month or more before it is properly firm all the way through. You cannot rush it. Poke it too early and you leave a mark you have to knife out again.
The hybrid polymer behaved completely differently. It tooled off firm, held its shape straight away, and was cured through in a fraction of the time rather than skinning on top and staying soft below. On a job where the window has to go back in the same day, that difference is the whole point. There is no waiting around for a skin to form before you can move the sash.
Paint time compared
Cure time matters most because of what comes next, which is paint. Paint is what protects a glazing line from the weather, and neither material lasts long if it is left bare. We cover why in paint is what makes glazing last, not the putty.
With linseed putty you have to wait. Painting over putty that has not firmed up traps the oil, wrinkles the paint, and can stop the putty ever curing properly. The traditional guidance is to leave it a couple of weeks at least, and longer in a cold winter. For anyone trying to finish a window over a weekend, that is the frustrating part.
The hybrid polymer let us paint far sooner because it was already cured rather than waiting to harden from the outside in. That collapses the job from something spread across a fortnight into something you can bed, tool and paint in a much tighter window. If you glaze in the cooler months, this is the difference between a window finished and a window sitting half done while you wait on the weather.
The 20-year outlook
Short cure time is only worth having if the result lasts. Linseed putty done well and kept painted can last decades, and that is exactly why it is still around. The problem is the failure mode. As putty ages it dries, shrinks and goes brittle, then it cracks away from the glass and the timber, and once water gets behind it the rot starts. Most of the rotten sash bottoms we cut out began as a cracked putty line nobody repainted in time.
A hybrid polymer stays flexible. Glass and timber move at different rates as they heat and cool, and a material that flexes with that movement is less likely to crack and let water in. It bonds to the glass and the timber rather than just wedging the glass in place. Kept painted, the expectation is a glazing line that flexes instead of cracking, which is the failure we see putty fall to over the years.
We are honest about the fact that hybrid polymers have not been on windows for a hundred years the way putty has, so nobody has a century of proof. What we can say is that the way it fails, or does not, is a better fit for how a sash window actually moves.
When putty still makes sense
We are not going to pretend putty has no place. There are good reasons to reach for it.
- Heritage and conservation work where the finish has to match the original method exactly, and a putty line is part of what is being preserved.
- Jobs where you already have putty on the shelf, the weather is warm, and you are in no hurry to paint.
- Situations where a client specifically wants the traditional material and understands the cure and paint timeline that comes with it.
For everything else, particularly a homeowner who wants the window back in and painted without losing a fortnight to cure time, the hybrid polymer is the easier material to live with. If you want the background on why we moved to it, we set it out in why you should ditch traditional glazing putty for a hybrid polymer.
The product we used on the hybrid side is our Sash Glaze Instant Adhesive. If you are reglazing from scratch and want the glass, setting blocks and tools together, the Reglaze Window Bundle has the lot in one go.
FAQ
Can you paint over hybrid polymer sash glaze sooner than putty?
Yes. Linseed putty has to firm up before it can be painted, which traditionally means waiting a couple of weeks and longer in cold weather. A hybrid polymer cures through rather than skinning on top, so it is ready to paint much sooner.
Does linseed putty still work?
Yes, done well and kept painted it can last decades. Its weakness is that it dries, shrinks and cracks as it ages, and once the line cracks and nobody repaints it, water gets behind the glass and the timber starts to rot.
Why does hybrid polymer last longer on a moving window?
Glass and timber expand and contract at different rates. A hybrid polymer stays flexible and bonds to both, so it moves with the window instead of cracking away from it, which is the failure putty tends to fall to over time.
Do I still need to paint hybrid polymer glazing?
Yes. Paint is what protects the glazing line from the weather regardless of which material you use. The difference is you can get to the painting stage far sooner with a hybrid polymer.
Is there lead in old putty?
Old putty and the old paint around it on a heritage window can contain lead. Wear gloves, keep the dust down, and wash your hands before eating or touching your face when you scrape it out.