Heritage window refurbishment in Launceston
A heritage window refurbishment in Launceston on this Lutyens designed extension , in particular, most of the putty was cracked and needed replacing along with several panes of glass. The oak sashes and frames had been painted black, and the putty picked out in white, and this style needed to be reproduced in the painting.The laborious way to do that would involve cutting out some of the defective putty and replacing it, then painting the window black and picking out the putty in white with a small brush…..there had to be a better way.
A lot of the major paint manufacturers do not recommend traditional linseed putty to face off the front of glass that is fixed from the outside for use with their paints. This is not only because modern water-based paints don’t like oil-based putty, but also because modern paints are micro-porous. This means that air and moisture are able to pass through the dried paint film, helping to prevent a build up of moisture behind the coating which could lead to early paint failure. Unfortunately, It also allows the linseed oil in the putty to dry and harden. This makes the putty brittle, leading to small cracks appearing in the fillet of putty that ultimately allow rain-water to seep in. The wood expands when it gets moist which loosens the putty further, as well as making a nice home for algae and fungus to grow. For these reasons, paint manufacturers would prefer you to use wooden beads or polymer sealants to glaze window panes.
So in this case, a lot of the putty was cracked and needed to be replaced. I decided it would be easier to remove all the putty and replace it with a polymer putty substitute, which rather handily comes in a white colour, negating the need for it to be painted white with a little brush!
Polymer putty substitutes have been available for many years now, and seem to me to come in two flavors. First is a water-soluble one that is basically like a painters acrylic caulk with fillers in to make it stiff and bulky. This stuff is fairly easy to use and, being water-soluble, easy to clean off tools, hands, and surrounding paintwork with a damp sponge. The downsides are you can’t use it if there’s a risk of rain before it’s dry, and also the drying process involves water evaporation which causes the “putty” to shrink slightly leaving a concave surface on the fillet which doesn’t look particularly good.
The second flavour is made from a modified polymer, which cures by chemical reaction with moisture in the air rather than evaporation so no shrinkage. The downside with these is that not only are they more expensive, but also very sticky, messy products to use. It takes a bit of practice to be able to produce a nice smooth flat finish that resembles the one that would be achieved by a glazier using traditional putty. I chose to use a new brand – 3C sealants glazing putty, a good value silicone based polymer sealant new to the market.
Having removed all the putty, I did the necessary repairs, applied a wood-preserver, and then coated the window up in Zinsser Allcoat in Black Satin before redoing the “putty”. These sealants can be smoothed with a filling knife, but I find it much better tooled with a Palu Tooling block. Some tradesman may be more expert than me, but I find the knack is to hold the block at a particular angle so that the excess sealant comes away cleanly and doesn’t smear over the glass or surrounding woodwork. Any excess that does find its way onto the glass is better left until its cured and then pared off with a razor blade. I found the 3C sealant good to use albeit a little more grainy than other brands I have used in the past, and so you don’t get a shiny smooth surface. In a situation where it will get painted, this may actually be beneficial, but in my particular case I have planned to leave the sealant unpainted (because it’s white). A dull surface may make the surface more prone to collecting dirt or algae in the future and is something I will need to keep an eye on (might end up painting it after all!). However, the sealant tooled very well and skinned over quickly, and I will definitely be using it again next year.
Published by: Colin Taylor on: Sep 13th 2017