The Good Oil – External Timber Coatings

When I started to think about what to include in this guide to external coatings I was originally just going to focus on the main types of coating options, however it soon became apparent that to understand what coatings you should use, you had to consider a wide range of other factors and just focusing on the coating could be misleading and unproductive. So I decided to extend the scope and take a more holistic approach – which has resulted in a short series rather than one issue.

What we will look at will cover:

Why use coatings at all-after all aren’t timbers durable?

How does design influence the use of timber coatings?

What is a coating and what are the differences, and

What coatings are best with what timbers things to look out for.

Why use coatings at all?

Generally finishes on timber have a dual purposes – firstly to improve the durability of the timber and secondly to add to the aesthetics. Unfinished, unprotected timber will degrade as a result of gradual changes to its physico-chemical structure brought about by UV, temperature and moisture.

The weathering process leads to a slow breaking down of the surface fibre resulting in change in colour and surface degradation. Under more extreme conditions, timber may deform & split or pull away from its fastenings. Different timber species perform differently when exposed to weather and no two situations will ever be identical, even to the extent that no two pieces of timber will perform in exactly the same way under identical conditions because being a natural product, each piece is unique, shaped by its growing environment, its location within the tree, how it was cut and even on what side of the tree that piece of timber grew on.

Although unprotected timber has been used externally for centuries, the weathered effect is not always desirable and in most applications timber needs protection from the elements to achieve a long service life. In addition to protection finishes enhance the aesthetic effect of the timber which will not remain unchanged if exposed to the elements unprotected.

The performance of exterior finishes depends on a wide range of factors including material type, quality and ingredients, however the local environment is a variable which needs special attention, as does an understanding of the coating types and their effect on the timber substrate.

Understanding the maintenance requirements to keep the timber aesthetically appealing, and thus how all these things impact on how and where the timber is used is also important.

Often there needs to be a compromise between the desired aesthetic and the maintenance program required to achieve this when selecting not only the coating products but also the timber itself.

The selection and correct application of the most appropriate finishing product is also a very important component of the design and construction process and understanding the various attributes of coating types is a good place to start, but there are other considerations such as understanding what coatings work best on what timbers, preparation of the timber substrates and the actual application process.

Timber is essentially a variable substrate, even within a single species. The density, moisture content, absorbency, flexibility, durability 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 finishing system must be sufficiently flexible to cater for this movement – so whilst its not as simple as you may have first thought, there is a way to work through the process to ensure you match your design with your timber and then with your coating system to get the best result.

Design – Timber is impacted most heavily by exposure to UV and moisture. If your building design can limit this exposure, you will have gone a long way towards improving the performance of your feature timbers and minimising maintenance.

Avoid having timber on north facing walls fully exposed to weather and also consider access to allow maintenance of the timber used – if its difficult or expensive to access, it is less likely to receive the maintenance it needs and your beautiful timber will soon lose its appeal.

Even those that favour the weathered timber look should understand that timber still requires protection and some maintenance or it will degrade under constant exposure cracking, twisting and splitting.

Covered timber decks, being protected from sun & rain, typically require minimal maintenance to retain their appeal and they are usable in all seasons
Clever integration of timber on south facing walls which are also partially protected from weather shows a stunning use of timber with reduced maintenance requirement.
And access to maintain the timber is easy as well
Although impressive initially, this timber facade will soon require maintenance which will be expensive and difficult to do. Multiple stories and high levels of exposure to weather will result in uneven weathering with some timbers going grey and others retaining some colour.

In the next instalment we will look at coating types and their pro’s and cons – in the meantime some examples of things to consider are shown below.

Consider the exposure to UV on fully exposed timber cladding and the maintenance impacts of coatings. Oils may need reapplication every 6-12 months
A very good application for timber where strong aesthetic impact is combined with low exposure, thus low maintenance.
A commercial cladding very exposed and in a high vehicular traffic area. Consider the difficulties associated with simple maintenance such as cleaning, as well as longer impacts of coating degradation and silvering of the timber.

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