It is not as though we can't do beautiful clear finishes over wood! Think of varnished floors, polished panelling, and sumptuous piano finishes – but all of these are indoors. Take them outdoors and their beauty (and our technology) simply fades under the influence of the weather.
To understand why, one needs to look at the marvellous nature of the wood itself. Using mainly water and carbon dioxide as building materials (along with some nitrogen and a smidgen of catalytic and essential minerals obtained from the soil), the tree builds wonderful structures that can support columns of water tens of metres high and cantilevered boughs that seem to have no visible means of support.
The major structural element that the tree crafts is a very long, straight chained sugar called cellulose. These molecular chains bond, one with another (using the same electrical forces which hold water together as a liquid when logic says that it really should be a gas) to form tiny fibres. These fibres are then formed into long tubes (complete with a, more or less, one way valve system), using a variety of lower molecular weight sugars and the more aromatic material, lignin. These lower molecular weight 'binders' are somewhat water sensitive so the whole structure needs a waterproof raincoat provided by the lignin-rich bark.
Just a little more! During a year's cycle, a tree's growth is not uniform, putting on the most growth in the spring and tapering off in the autumn. This is reflected by lower density, larger diameter tubes in the spring wood and higher density, narrower ones in the latewood bands. Further, the tree only needs a few tens of millimetres of 'sap carrying wood' in order to supply its wants so, as fresh layers are created every year, the vestigial material morphs into more resin rich heat wood, which enhances a structural core.
So there you have it! A pretty sophisticated pillar made up of flexible annular laminates around a more rigid core.
As mentioned earlier, wood needs to be protected from the elements by its sheathing of bark. When this is removed, as when it is milled, the lower molecular weight binders become exposed and degrade under the influence of water and, especially, U.V light. The cellulose fibres become unbound and can be seen as a silvery grey mat upon the surface – or become the home for black mould!
Slower growing hardwoods typically show less seasonal variation along with greater durability.
(N.B. This memo refers to durability specifically in terms of resistance to water and U.V. light – not resistance to mould, fungi or insect attack).
The mode of failure of a good varnish film, over wood and exposed to the elements, is as predictable as death and taxes! Moisture is ubiquitous and, with the amount of U.V. light getting through the film, will degrade the binders holding the cellulose together. The first visible sign will be a slight lightening in colour as the film starts separating from the substrate. Invariably it will start on areas of spring wood and on flat sawn boards.
Very soon the separated film will crack, allowing more water and mould spores to enter this space. Degradation of the system proceeds apace, resulting in flaking. No matter how good the varnish, how much U.V. absorber one loads into it or how good the timber is under it, time is on the side of flaking.
Flaking is simply an unacceptable form of failure, primarily due to its patchy nature. Enormous effort is required to return the surface to the uniformity required before it could accept another clear finish and only a madman or a boatie would sentence themselves to a Sisyphean life of endless sanding.
It is well known that pigments are excellent at absorbing U.V. light along with most of the visible spectrum. Load up a varnish with such pigments and you get – well you get a paint actually and that is clearly not clear!
by Colin Gooch, Resene Technical Director
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.