Architects memo no.
30: July 1983
applied finishes for timber - exterior situations
in New Zealand
Before developing the main theme of this memo it will be advantageous
to consider the timber surface which is to be coated.
Most timber is made up of cellulose fibres arranged in long, slender,
hollow tubes or cells. The cellulose fibres are bound together with
lignins, hemicellulose, and a wide variety of compounds lumped together
under the term 'extractives'. The thickness of the cell walls and the
ratio of cellulose to lignins varies with seasonal growth so that the
surface of any piece of timber is, by nature, a non-uniform substrate.
Timber is in general a very stable material retaining its mechanical
and chemical properties over long periods of time. It will however,
undergo cyclical changes of expansion and contraction with heat and
cold: and swelling and shrinking with the ingress and egress of moisture.
This dimensional instability with moisture varies from species to species
and has been correlated with the 'paintability' of individual species.
The chemical stability of the timber surface is poor under the influence
of UV light. Under such circumstances wood loses methoxy groups and
lignin, with a corresponding increase in acidity and the evolution of
formaldehyde and methanol. Loss of these elements creates voids in the
timber which results in enlargement of the cell orifices and cracking
of the cell walls. Areas of high lignin content are more prone to UV
damage than areas of low lignin content. These chemical changes are
accompanied by a colour change towards grey and the exposure of loosely
bound cellulose fibres on the surface of the timber.
The prime considerations for a coating for timber, therefore, must
- Moisture impermeability
- Ability to resist UV degradation of itself and to screen the timber
- Heat reflection
It is assumed that the timber requiring coating is in ideal condition,
i.e. it has had the correct pretreatment commensurate with the projected
exposure conditions; it has been dried to the correct moisture content;
it has been carefully stored and is free from surface checking; and
has been correctly detailed to avoid sharp arrises and allow free drainage
of water. The next all-important step is in the choice of a primer.
The principal requirement of a primer is that it adheres tenaciously
to the substrate both initially and long term (ideally the life of the
structure). Other properties that are desirable in a wood primer are:
- Long term flexibility.
This function is more easily achieved with saturated high polymers
(such as acrylics) than unsaturated vegetable oils and alkyds. Within
the oil-based varieties, oils with the least degree of unsaturation
(whilst still allowing air-drying), have the potential for longer
term flexibility than the more unsaturated oils such as linseed.
- The ability to make uniform the porosity of the non-uniform substrate.
It is accepted that the porosity of timber surface varies widely on
the same piece of timber, and the need for the primer to cope with
this can lead to some archaic concepts. The phrase 'oil-based paints
feed the timber surface' has for many years 'coloured' the thinking
of the primer function. Though penetration into timber pores can occur,
it takes place in a selective manner, usually the solvent first followed
by low molecular weight vehicle fractions and so on as viscosity increases
and drying speed permits. This then can also lead to lack of uniformity
in the primer surface. It is the author's belief that penetration
should be reduced to a minimum by formulation, and uniformity of surface
achieved by a tightly adhering primer film on top of the timber surface.
- Water resistance.
Accepting that water is one of the most severe degrading influences
on timber, it is obvious that the ability of the primer (and the rest
of the system) to keep the timber dry is beneficial. However, totally
waterproof coatings are rarely a practical proposition and it must
be accepted, even in a well coated system, that the moisture content
of the timber will vary with subsequent selling and shrinking. There
is a case for inducing into primers a degree of hydroplasticity such
that it can move sympathetically with the timber.
- Stain resistance.
Timbers, according to species, contain extractives that can migrate
through paint films causing objectionable staining on the surface.
It is yet another function of the primer to prevent or reduce this
phenomenon. Since the majority of the extractives are water soluble,
the problems are most noticeable (but not wholly confined to) waterborne
primers. Reactive pigments and ion-exchange type resins are useful
in reducing these problems, as is the careful screening of the water-phase
components of the primer. In the Author's experience there will always
be one bit of timber that will beat the system!
- Fungal resistance.
The timber surface is often a happy hunting ground for moulds and
fungi. Hoffman has shown that increased fungal resistance occurs when
fungicides are incorporated throughout the paint system rather than
concentrated in the topcoat. A good timber primer should show fungicidal
- Corrosion resistance.
Whilst it may seem strange discussing corrosion resistance in relation
to wood primers, it is a fact that the appearance of many otherwise
satisfactory paint jobs can be marred by the presence of rust stains
derived from nail head corrosion. Wood primers should assume the job
of combating this.
The primer must present a surface suitable for recoating.
Wood primers in New Zealand are traditionally a shade of pink (even
though the base may be white) notwithstanding considerable pressure
from the painting trade to have them white. The painter's reasons are
obvious - the manufacturer's not so. The primers are shaded pink so
that they do not contribute to the hiding of the system. A contribution
to the hiding implies a contribution to the absorption and reflection
of light including the damaging UV component. In the interests of maximum
durability of the primer, this is not desirable. As listed above, the
primer has sufficient functions to perform as it is.
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