The "greening of England" 4

Apart from its ‘natural’ origin, timber as a construction material differs in one fundamental way from both plastic and aluminium: That is, of course, the finishing process.

Aluminium is anodised before it is cut into components and assembled into frames, whilst pvc comes out of the extruder as a finished product.

Much of the focus of the production process for the latter materials is in preventing damage to the finish, as repair is always difficult and expensive, requiring that elusive skilled labour again.

Timber, on the other hand, is traditionally worked in the raw, and the final finish is applied to assembled frames, sometimes after the hardware is fitted. Most unsatisfactory.

To complicate matters, there has been a sea-change in the way wood-finishes themselves work. Although micro-porous finishes have been around for more than forty years, until about twenty years ago traditional oil-bound paints were still predominant and in common use. These were impermeable, sealing the timber and to a great extent limiting the seasonal movement of moisture in and out of the timber. They were usually applied on-site by a decorator who might or might not remove the hardware first.

The main drawback was their tendency to split and flake, letting water in. That couldn’t get out, and once in, it encouraged fungal growth in the wood under the paint, which blew more of it off. When micro-porous translucent finishes became common during the 1980s, they stopped all that, and transformed the maintenance requirement. The downside was that seasonal moisture changes led to massive dimension changes, too. The designs had to accommodate this. No problem with modern styling, but more difficult to retain a true traditional appearance.

The next big change was from solvent-borne finishes to water-borne, for Health and Safely reasons. Like all things in life, there were advantages and disadvantages.

The one big advantage of water-borne finishes is the short drying time. Minutes or hours instead of days (especially in the winter). Another is the thick film-build possible in each coat. The downside is the relatively poor stain penetration, so the underlying colour of the wood influences the final finish. This can vary from piece to piece, or even from place-to-place in some timbers. Water based finishes don't "wet" the surface of the wood as well as white spirit, and can leave the open-grain hardwoods with a fine pitted appearance. In reality, they are fine air-bubbles that need heavily overcoating to close.

Although the technology is improving all the time, it is true to say that the first five years of the change-over period of the mid-90s were a continuous nightmare. Frames sticking to each other, even a week after they were sprayed: Little runs and sags that simply could not be touched-up, but needed stripping and refinishing: Blotchy, uneven colours, and much, much, more.

I am very pleased to report that these problems are now a thing of the past, but, believe me, it made staying with wood windows difficult at times.

Recent developments make it all worth while…..

More next week.