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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 with clears.

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:

  1. A significant improvement in UV resistance.
  2. An equally significant improvement in fungal resistance properties, and
  3. 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.

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