Architects memo no.
36: May 1984
clear timber finishes - now is that perfectly
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:
- They cannot reflect heat, as any heat reflecting pigments will reduce
or eliminate the clarity of the coating.
- The path through the film for moisture is very short and unimpeded
due to the absence of pigments.
- A clear coating designed to allow the unrestricted passage of visible
light cannot entirely stop the passage of closely related (and damaging)
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.
- Cracks developed in the coating/timber system due to moisture penetration
and swelling of the timber.
- Rapid attack by UV and moisture at the site of the crack.
- Colonisation by mould at the timber interface and further sideways
- Loss of adhesion around cracks resulting in "peel-back" and discolouration
of the timber.
- 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.