Architects memo no.
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
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
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.