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Timber performance

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Substrates

As a substrate timber is variable in its performance, even within a single species. The density, moisture content, absorbency, flexibility and the nature of its extractives may vary considerably.

Some timbers are also more susceptible to dimensional change due to moisture or humidity variations. The paints and stains must therefore be sufficiently flexible to cater for this movement.

Differences in paint performance have been substantially overcome with the introduction of 100% acrylic polymers. Alkyd or Enamel paints have a tendency to embrittle on ageing (that is, those that are solvent borne). They may ultimately crack and peel away from the harder grain surface despite having good initial adhesion.

Uniformly fine textured timbers have better paint holding characteristics than coarse textured species. The latter respond better to penetrating stain applications.

The way timber is cut from a log affects the orientation of the growth rings; this in turn affects its paint holding properties. Back-sawn softwood boards have wider, darker bands of latewood than quarter-sawn boards, the latter tending to hold paint better. In hardwoods the difference is comparatively small compared to that of softwoods.

If a board is quarter-sawn (edge-grained) as opposed to back-sawn (flat-grained), its dimensional stability is better under variable moisture conditions and it is less likely to result in surface checking to the detriment of the paint system.

However most parcels of timber are supplied as a mix of back-sawn and quarter-sawn pieces.One hundred percent acrylic paints can cope with this variation.

Discolouration or blistering of the paint may be caused by extractives of the heartwood such as gum or resin. This effect is accentuated by heat and is more common on the side of the structure receiving maximum sunlight. Kiln drying of the timber and better selection by the wood processors have largely minimised this issue.

Some timbers have a higher percentage of aromatic oils, which contribute to their characteristic pleasant odours. But these oils may cause problems with paint discolouration and inhibit drying if left untreated before painting. Check the paint, stain or varnish chosen is suitable for the wood type. Generally acrylic lacquers (solvent or water based) don’t suffer from poor drying when applied over oily woods.

Where plywood or glued laminated timber is used, an exterior timber grade must be specified together with an appropriate paint system.  Note – system means multiple coats designed by the paint manufacturer.

  • In general, all timber types can be expected to absorb and release moisture from day to day.
  • Paints and stains do allow the movement of water through the film in the vapour phase.
  • Paints and stains contribute to slowing down the transfer of that moisture, which minimises the expansion and contraction of the timber, leading to a more stable substrate.
  • It is the rapid up take and evaporation of moisture that causes the timber to be stressed, which can lead to cracking of the wood and paint failure.

Some substrates such as MDF specified for interior use can be damaged permanently through water absorption. This causes irreversible swelling of the grain. Therefore even with interior situations, proper application of the correct number of paint coats goes a long way to ensuring good performance of the wood.

Substrate VariableEffect on Performance
SpeciesDiffers between and within species.
DensityThe heavier, denser timbers are less prone to moisture uptake than lighter species.
Dressed TimberBetter performance with conventional paint systems.
Sawn timberBetter performance with solid or semi transparent stains.
Timber edgesSharp edges create stress on paint films and make it difficult to get the correct film build, causing failure. Arised (bevelled) or rounded edges permit film movement.
Weathered versus
un-weathered timber
Surface deterioration of timber fibres due to weathering can result in poor paint adhesion unless corrective sanding is carried out. The black look comes from fungi growing on the decayed fibre. For both paints and stains, remove the decayed matter by sterilising and sanding back to new fibre. Then paint or stain as if you have a new substrate.
ExtractivesExtractives are mostly wood resins such as gum rosin. Wood with high knot content should be prepared knowing that resin bleed is possible. There are several paint products designed to restrict resin bleed. Don’t short cut on the amount of paint used on knots, several coats will work better. Make sure they are thoroughly dry before you apply your top coats.
Heartwood vs
sapwood
Heartwood has a higher staining potential than sapwood. Staining of paints generally comes from the tannins contained in the wood. Most 100% acrylic paints are designed to restrict or capture those tannins. If a stain does appear on the first coat it is best to let that area dry for a few days before proceeding with the next coat. This gives the film a chance to lock in the tannin and restrict its future migration. In extreme circumstances you may have to use a paint that has been specifically designed for stopping stain and tannin movement through the film.
Smooth texture vs
coarse texture
Smooth texture surfaces have better paint holding potential than coarse texture ones. Use of stains on the latter gives good performance.
Gum pocketsCan lead to resin exudation and staining unless pre-treated and sealed. When selecting your wood note that boards with high knot content are generally of inferior quality. While you may save on the cost of the initial purchase you can expect to spend more on your paint costs through the use of knot sealers and fillers, and can expect higher painting maintenance costs.
Aromatic oilsCan lead to drying retardation and staining if surface oils are not removed. Some wood oils can retard the drying process of alkyd/oil type paints and varnishes. This is particularly true of native timbers.

Check with the recommendations of your paint brand when painting native timbers. If in doubt use acrylic primer / undercoats.
KnotsPossible premature cracking, staining and resin exudation can occur unless treated with knotting sealer (even with knotting sealer one cannot guarantee the resin is completely sealed off). Apply the knot sealer before you apply the filler, use several coats of knot sealer and let it dry thoroughly before applying your top coats.
Moisture contentNormal range 10-15% - higher levels detrimental. Generally when the EQM (equilibrium moisture content) is greater than 16% painting should not proceed.

It is impossible to tell what the EQM is without a moisture meter.

As a guide, wood left covered in a dry environment and stored off the ground will have an EQM level associated with local conditions but mostly below 16%.

Timber that gets saturated by rain (left outside uncovered or lying on the ground) even with a pre primer will get too wet for painting to proceed.

This is particularly important for exterior pre-primed weather board as shrinkage post painting of the boards can occur as the timber dries out and stabilizes.

Most pre-primed weather and facia boards have instructions either as a sticker attached or via data from the manufacturer’s web site. Follow them closely; it’s not that hard to protect the boards from excesses of water contact prior to installation.

Once installed, and where possible immediately, prime all bare areas and complete the application of putties, sealants and flashings.