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:
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.
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.
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.