Surface treatment of outdoor timber

By | 2019-09-06T13:18:36+00:00 September 6th, 2019|

By Merensky Timber

With the dwindling supply of tropical hardwoods caused by major deforestation of natural forests, and the subsequent high prices of sustainably-sourced tropical hardwoods, there is a need for alternative sources of wood that don’t carry a hefty price tag – for the user’s pocket, or for the environment.

Merensky walkway exposure testing of various surface treatments under high foot traffic conditions. All images by Merensky Timber

Merensky walkway exposure testing of various surface treatments under high foot traffic conditions. All images by Merensky Timber

There are many alternatives that can be used for interior applications, but woods suitable for exterior use are a completely different proposition, as explored in this article.

Merensky Timber is researching exterior-use wood for various reasons. “This allows us to utilise our fifth-generation Eucalyptus Grandis hardwood to its fullest extent and to provide existing customers with practical advice on the use of timber,” says George Dowse, researcher at Merensky Timber. “We offer affordable, sustainably-sourced alternatives to tropical hardwoods, and place much emphasis on testing the various options in our South African climate, which differs considerably from the climatic conditions in many of the countries where large-scale tests are typically performed.”

Surface-treatment testing

Wood used in exterior applications such as decking or walkways needs to be both strong and durable. One option for improving timber durability is to alter its physical properties through impregnation, heat treatment, or a combination of these processes. This works for certain applications but in most cases the overall costs end up as high as for tropical hardwood alternatives, and producers require large capital investments.

“With this in mind, Merensky has turned its focus to surface treatments, and understanding what best suits our timber in various applications, so that we can give practical advice to our customers,” says Dowse. “Our exposure testing covers the

majority of South Africa’s commercially-available surface treatment options, including oil-, solvent- and water-based alternatives. Our testing also covers combinations of these as well as the use of additives such as waxes and UV-protective colour pigments.”

Dowse says the Merensky Research team has drawn up various conclusions through this testing, which form the basis of its recommendations to customers.

Location and climate

Samples tested and exposed in Cape Town (Western Cape) degraded much faster than those tested and exposed in the more tropical Tzaneen area (Limpopo province). Treatment selection therefore needs to take into account environmental factors such as humidity, wind, UV index, rainfall and wet seasons.

Merensky walkway after 18 months’ exposure and a fresh maintenance coat.

Merensky walkway after 18 months’ exposure and a fresh maintenance coat.

Application

Certain samples that weathered very well on exposure racks did not fare as well during walkway tests, when subjected to everyday use and wear and tear. Factors that play a role here are physical abrasion (such as foot traffic on a walkway or deck), orientation (whether the surface is flat or vertical, in the sun or shade) and construction (such as exposed areas at joints, or moist areas under decks).

Wood quality

During testing, our higher-quality or select-grade lumber was identified as least likely to exhibit surface checking, splitting or cupping – all features that increase the risk of water ingress, which has a damaging effect on timber. Exposure to the elements, as well as drying and treating the wood, can further develop these defects, especially in lower-quality timber.

Type and density of wood

Not all treatments can be universally applied on all species and variations of wood. The Merensky exposure tests revealed that some treatments formed a good-looking surface layer after treatment but did not penetrate the wood surface layer – especially on higher-density hardwoods. These may therefore start to flake quickly when treatments dry out or are exposed to high-wear conditions.

Maintenance requirements

Film-forming surface treatments such as varnishes were shown to start flaking or peeling when not maintained for prolonged periods and, in most cases, a complete removal of the treatment was required in order to remedy this. Most manufacturers recommend that a fresh coat be applied, either within a certain timeframe (for example, within 24 months), or when the surface appears dry.

Penetrating treatments such as oil-based products are similar, but with one key difference: they are far more forgiving when not timeously maintained, and a fresh coat can usually be applied without the need for any remedial action. In such cases, though, the wood’s appearance might have changed, depending on how long the surface remained unmaintained.

Certain treatments were shown to provide very good alternatives to regular oil or varnish products, with regard to both penetration and maintenance requirements. Significant advancements were demonstrated with penetrating sealers and water-based treatments.

Test samples from the exposure racks after 18 months’ exposure showing clear signs of flaking due to a lack of bonding to the hardwood surface.

Test samples from the exposure racks after 18 months’ exposure showing clear signs of flaking due to a lack of bonding to the hardwood surface.

Film-forming vs non-film-forming

In addition to the maintenance requirements of different surface treatments, users should be aware of the differing effects of film-forming and non-forming treatments on wood.

A film-forming treatment can prevent moisture from penetrating the timber and therefore help maintain its original aesthetic appearance for as long as it is regularly maintained. If the wood is not maintained, however, or surface wear causes flaking, its appearance can very quickly deteriorate.

Penetrating treatments may allow in some moisture, but, because the wood is already penetrated by oils or waxes, the effect on the wood’s properties is minimal. The surface might discolour over time, but the structural condition of the wood remains intact.

Factors to consider when choosing a treatment

Ease of application should be considered when considering surface treatments, as some products are not easy to apply with a brush or roller.

The initial treatment cost, in addition to maintenance costs, should also be considered. In some test cases, the more expensive treatments did not perform better in the long run.
The aesthetic appeal of a treatment should also be considered. Gloss and matte-type finishes are available for most product options and generally have similar protective properties, but a glossy surface might not stay glossy in certain applications.

A very high-density tropical hardwood showing the importance of matching surface treatments to the species being treated.

A very high-density tropical hardwood showing the importance of matching surface treatments to the species being treated.

Find the best treatment option

  • Hardwoods and softwoods often require different treatments. A reputable surface-treatment supplier should be able to advise you in this regard.
  • Keep to the supplier-recommended maintenance schedule. This will ensure a longer-lasting, good-looking product that will ultimately be less costly to maintain.
  • Naturally durable hardwoods should be used for applications where the recommended maintenance schedules cannot be met.
  • For higher-exposure classes, such as protection against insects and in-ground or water applications, additional impregnation treatments are required.
  • Timber is a natural product and therefore cannot be used in all applications.