What Is The Best Glazing Putty Or Caulk To Use In Restoring 150 Year Old Windows?


3 Answers

Drew Skuce Profile
Drew Skuce answered
First off.

There is no product in existence today that can replace Traditional Glazing Putty.
The material is designed to be a flexible bond between 2 very dissimilar material for many many many years.
Now you may say that Caulking does just that....sure....maybe for 10 years if you're lucky. Then it has to be replaced after it fails horribly.

Now I know of a building in the Eastern seaboard that has the ORIGINAL putty still intact since the 1640's.
The key is that putty needs new oil added to it ever 5-7 years to keep it flexible. If that is done...there is no reason that putty won't last 1000 years.
It is a fact that the Nixon family (yes...THAT Nixon) home has records of the window glazing putty being re-oiled every 5 years for the last 300 years. It is still in near-new condition. And that's afters centuries of rain and winters being thrown at it.

I don't know of a SINGLE Caulk-type product that can boast a track record even close to that.

Now Real glazing putty is a mixture of high calcite Lime and Boiled Linseed oil (higher grade the better). It is NOT Clay.
The higher the Calcite content...the better. Ie: Less than 5% Magnesium content.
How it Dries and Cures are 2 very different things.
First off...the Oil is a drying oil. It Oxidizes and dries in the mix after about a week of environmental exposure.
Next is the Cure process. This happens when the Lime is exposed to the Carbon Dioxide in the air. The Lime reverts back into it's original Calcium Carbonate (Limestone) form in about a Year time, if it's painted with a Latex. If it is sealed with an Oil based paint. Full cure time is stretched to about 30 years.

Now you want the Putty to remain flexible to continually compensate for the 2 dissimilar materials expanding at different rates. So the longer it takes to fully cure...the better.
Hence why the 5-7 year schedule of re-applying linseed oil is such a good idea. It keeps the putty flexible for centuries.
Just paint on a layer of linseed oil right over the painted glazing surface. It will soak right in and do it's job.

Note: A little bit of a note about the wonder-material of Lime is that it's Self-Healing. If any Micro-cracks form in the lime, they heal themselves closed again. Because that micro-crack is then exposed to the air. The lime absorbs carbon-dioxide and carbonates faster and grows tiny calcium carbonate crystals to fill the crack.
There aren't any Caulk products on the market that Self-Heal.

Now. You ask how to REMOVE old putty that has finally failed after a 100 years of no-maintenance (which is why most people get rid of their heritage windows).

Simple: A little mixture of House-hold Chlorine Bleach and Raw Linseed oil.
Put it on the window glazing putty (with the window flat on a table) and let it soak in for about a day.
The Raw Linseed oil will start to work away at the oil-dried out Boiled Linseed oil and start to re-emulsify it.
The Chlorine bleach will start to attack the lime and break it down.
Then take your Roller-Chisel ....

www.swedepaint.ca target="_blank" rel="external">www.swedepaint.ca To ride on the flags of the muntin bars to cut out the old putty.
(I am not affiliated with Swede Paint...it's just the only place I know that sells the chisel)
Heating the old putty is a dangerous way of just reactivating the old linseed oil. Most people will have greater than a 30% loss rate of the glass.
Heat-Guns in general should be banned. They output far too much heat for any practical use. They destroy glass and cause the off-gassing of white lead ALL THE TIME. They are also a great way to burn your house down.
Think of this for a moment. 1000*F hot air being blown into a crack around a window architrave and igniting a pile of ant chewed saw-dust. Wood spontaneously combusts at around 575*F.
It's something stupid like 20% of all house fires in the USA are caused by heat-guns.
If I remember right. White Lead based paint turns into lead vapour around 375*F and will kill you.

Heat-Guns are BAD.

Ok. Back onto something happier. Lol

Now when you put in new Glazing points (those triangle metal things holding in the glass)
Make sure you get some that are Zinc plated. The Zinc reacts with the lime and self etches the glaze points into the putty for a great bond.
Easiest way to tell if they are not marked on the package....is to look at the colour. Zinc has a slight Blue tint to it. Once you start seeing that Blue tint...you'll never miss it again.
Just like how Stainless Steel has a slight Yellow/Honey tint to it. Once you start seeing it...you Can't-Not-See-It. Haha.

Regarding DAP-33. It is not an appropriate product to be using. It takes Forever to dry to a point that is good to paint. It does not have super-longterm flexibility. There are no known chemicals/solutions that will remove it. It is also about the same hardness as Portland Cement once it does cure. It is a Nightmare to try and remove it once it has dried. Plus Good Luck getting it all out without breaking the glass. It's just too hard. It is also not self healing.
The only thing that it is a good for is application ease. It goes on almost the same as Traditional Putty. BUT beyond that it fails in almost all other respects.

Regarding Caulks. They are long term flexible (5-10 years..20 if you are Super Lucky). Application is not easy and clean-up is annoying at best. They also look like crap in a any pre-1700 to 1920 era window. That sharp-cut chamfered edge is what makes the exterior character of a heritage window just pop with class IMO.

IMO nothing can replace Lime/Linseed Putty. To me it's an absolute wonder-material that has been around for the better part of 1000 years. That's the kind of track record that I'm going to trust. Not some New Fangled Product in a tube that has had "5 years of simulated use" put to it. I'll pass on that.

Ontario Canada
thanked the writer.
Anonymous commented
So...I should mix my own "traditional glazing putty", of just high-calcite lime and boiled linseed oil? No other ingredients? And in what proportions do I mix them? And how long should I wait to prime and paint it? And should I used oil-base primer under oil-base paint, oil-base primer under latex-base paint, or latex-base primer under latex-base paint? Oil-base paint is really horrible stuff, because it cracks.
Anonymous Profile
Anonymous answered
First off the fact that you had such a failure rate indicates an application error. You have to treat the wood before putty application. An old method is to shellac the rabbets before glazing. I use linseed oil as a sealer because I can glaze right on top of it without waiting. If you don't seal the wood it will suck the oil out of the putty and voila! Putty failure.

Great info in that last reply. I have been restoring windows for twenty years and hadn't heard some of that . Wonder if the Nixon putty had been painted or just oil-maintained all those years.?

I have a friend who makes his own putty with calcium carbonate and linseed oil. He gave me some of his calcium carbonate to dry soupy putty with. He got his stash from a paint manufacturing company in his industrial neighborhood. I know hydrated lime is available for masonry applications. My friend mixes his with an old dough mixer. You can do small batches by hand or web search linseed oil putty.

As for heat guns being the devils tool: I use one all the time and I have found the secret to be, you guessed it, read the instructions. You need to keep the gun moving at all times. Never let it rest. Never hold it in that stubborn spot. You need to pre-heat the next spot you're headed for anyway so keep it moving. (But first try that cool trick with the linseed oil and bleach.) AND because you CAN start punky wood to smouldering very easily, always keep a garden hose handy. I also keep a two liter soda bottle full of water at hand. I would be very cautious about using a heat gun in certain situations and yes I have seen tragic fires in historic buildings caused by improper use of them but this is way off topic.

Good Luck,
Anonymous Profile
Anonymous answered
Scraping and sanding can diminish the value of old woods... And  I am a fan of the latex caulks as they dry pretty quickly.
This is what I found on glazing  www.stretcher.com

This is what I found on urethane caulks www.askthebuilder.com

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