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au natural

Architect's memo 67: September 2001

A nicely grained piece of timber is indeed a thing of beauty and there is an entirely understandable desire to leave it unadorned or, if coating is necessary, to use a clear coating system that maintains or even enhances its natural beauty. Indoors this is readily achievable but achieving equivalent results outdoors remains elusive.

In the external environment the enemy is UV light with its sidekick, the ubiquitous water.

The basic structural element of wood, cellulose, remains one of nature's polymeric masterpieces and is exceptionally durable. The 'glues' that bind the cellulose fibres into a cohesive whole are however somewhat less durable. Like sniper's bullets, bolts of UV light break down these binders reducing them to volatile fragments causing erosion of the surface of the timber.

Different timbers have variable resistance to UV. There is a simple rule of thumb to guide us in resistance predictions: - the denser the timber, the more resistant it is. Unfortunately some of the more favoured species (such as Western Red Cedar) are relatively light and correspondingly prone to UV damage. Even when derived from the same tree, the cut of the timber will affect the durability with, contrarily, the more handsomely grained flat sawn pieces being less resistant than the more boring quartersawn pieces.

Anywhere visible light goes some UV is bound to follow. Consequently a coating designed to reveal the grain will invariably allow some damaging UV through. While this can be reduced to a mere trickle by incorporating UV absorbers into formulations, nature has time on its side and damage will occur. Once the damage occurs, the clear coating suffers a fate akin to having the rug pulled from under its feet and flaking occurs.

Flaking is an unacceptable mode of paint failure no matter how many years of good service the coating may have given prior to the breakdown.

Nothing is completely free from this effect including the so-called 'marine varnishes'. Just remember that yachties are a breed apart, willing to put multiple coats on their bright work and scrape it off and recoat every year.

Pigments are designed to absorb or reflect light and their inclusion in a coating offers significant protection. The greater the amount of pigment, the greater level of protection. The problem is that with increasing pigment levels, coatings stop being transparent and become more paint-like.

Semi-transparent stains are something of a halfway house offering some of the aesthetics of clear coatings and some of the protection of paint. Their transparency allows some UV light to get through to the surface, which necessarily forces the paint chemist to develop formulations to prevent flaking.

The direction generally taken is to design a binder system that will erode away before the onset of flaking. This means deliberately reducing the durability of the film, and in some technologies, using carriers that do not even form films. It is a delicate balance to achieve maximum durability while still avoiding undesirable flaking.

This balancing act between aesthetic appearance and controlled erosion reduces the durability of semi-transparent stains compared to paints, necessitating a more regular maintenance schedule. The search is always on for newer technologies that can enhance this balance.

Areas that will need the greatest amount of maintenance will be north-facing, dressed timber, unprotected by soffits and coated with a lightly tinted stain.

Added durability comes with denser timbers, surface roughness, protection from the worst of the sun and coverage of the substrate with a heavily tinted stain. There are also successful brush applied pre-treatments on the market whose aim is to upgrade the UV resistance of the timber itself.

So fully clothed or 'au natural'? As always there is compromise and a carefully formulated semi-transparent stain can find the middle ground between beauty and practicality.

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