Architects memo no.
37: June 1984
improving the surface durability of timber -
it goes against the grain
In memo No. 36 we discussed the requirements for a successful
clear coating for exterior timber. It became quite evident that most
of the problems faced were due to the nature of timber itself. It is
only logical to examine the timber surface to see in which ways it can
in itself be upgraded.
Timber can be looked upon as bundles of fibre cemented together with
lignins. Lignin is a very imprecise word as lignins from various timber
species vary dramatically from one another, particularly in the area
of UV and water resistance. Because of this, some timbers are naturally
more durable than others, and form a much better basis for overcoating
The method by which timber is milled can also have a pronounced effect
on the surface durability; particularly the water absorption. Experiments
conducted on Radiata Pine using flat sawn and quarter sawn panels showed
that the flat sawn panels absorbed almost twice as much water as did
the quarter sawn.
This difference was apparent after one week's exposure and still existed
at the conclusion of the test. Obviously the less water absorbed, the
less water-damage to the timber will occur.
It has been observed often that rough-sawn timber is more durable
when coated than dressed timber.
Suggested reasons for this improvement in durability have been that
the roughness of the surface (a) holds more coating, and (b) caused
a 'shadow' effect that protects the coating.
Whilst both of these reasons probably do contribute to the effect
there is also a third likely contributor. Dressing timber puts a lot
of stress on a timber surface, particularly compression. This stress
must relieve itself in the fullness of time and probably contributes
to cracking of coatings and hence degradation. There is evidence that
this stress can be considerably relieved by wetting the surface with
water and actually inducing the 'grain raising' that is normally avoided.
This 'grain raising' is probably giving a valid picture of the compression
that was present on the surface.
Another avenue for improving the surface durability of timber is to
chemically treat it. Work was first carried out in America, and subsequently
in New Zealand, on the use of chromates and chromic acid as a surface
treatment for timber. The actual chemistry of what happens to the timber
surface when these strong oxidising agents are applied is not fully
understood but the practical effects are well documented. They are:
- A significant improvement in UV resistance.
- An equally significant improvement in fungal resistance properties,
- A decrease in the moisture uptake of treated samples.
Use of such pre-treatments upgrades the performance of subsequently
applied coatings, whether they are clear or pigmented.
Unfortunately these treatments do give a colouration to the surface
(a non-uniform green to greenish-brown to brown) which although acceptable
for some uses, does not meet with universal approval.