clear timber finishes - now is that perfectly clear?

Architect's memo 36: May 1984

Timber is one of the most inherently beautiful building materials known to us. It is only natural that people desire a coating for timber that preserves the infinite variety of grains and colour. Indeed these needs are met perfectly for timber used in an interior environment; the problems arise when timber is on the exterior.

In order to understand the reasons for this, one must first consider the surface chemistry of timber. The relevant information is contained in Architects Memo No. 30 paragraphs 4 and 5. Clear coatings do not easily match the requirements of a coating for timber because:

1. They cannot reflect heat, as any heat reflecting pigments will reduce or eliminate the clarity of the coating.
2. The path through the film for moisture is very short and unimpeded due to the absence of pigments.
3. A clear coating designed to allow the unrestricted passage of visible light cannot entirely stop the passage of closely related (and damaging) UV light.

Clear films however, can be designed to have excellent resistance to UV light in themselves and this, in general, was the tack that manufacturers took. Results are generally disappointing as UV resistant films failed in a classic pattern.

1. Cracks developed in the coating/timber system due to moisture penetration and swelling of the timber.
2. Rapid attack by UV and moisture at the site of the crack.
3. Colonisation by mould at the timber interface and further sideways attack.
4. Loss of adhesion around cracks resulting in "peel-back" and discolouration of the timber.
5. Simple clear oil treatments are not subject to this "peel-back" problem but they erode so rapidly that constant maintenance is required.

The route to achieving a satisfactory clear finish for timber must take into account the twin enemies of water and UV light. Experience shows that it is unlikely that a suitable coating polymer will ever achieve vapour barrier properties so one must accept that water will get to the timber and the timber will subsequently move. The aim must then be to achieve a film flexible enough to move with the timber and thus resist cracking. This tends to bar most thermosetting polymers.

The second objective is to design a binder system that not only is UV resistant in itself but has the ability to screen the timber from the damaging rays by absorbing them. This technology is not new and for years suntan lotions have been using organic UV absorbers. Such materials have been adapted for coatings and improvements can be shown. Unlike the suntan lotion, however, the clear finish ideally should last for some years and it is in the area of long term cost/performance that there is serious doubt about organic UV absorbers.

Recent years have seen the availability of some synthetic iron oxides pigments in so fine a form that they become transparent in vehicles. More recent work has shown that even though transparent to visible light, they remain very effective UV absorbers. They also have the advantage of being virtually permanent. The disadvantage is that they are coloured but at the use level the colouring is slight, so slight that it is virtually unnoticeable over timbers such as cedar and redwood.

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