Wood Works No. 6 – Wood finishes – which is best for my outdoor wood products?

There is a wide range of wood finishes on the market. Here, we take a look at the main types and provide some links to help you in your research

Can I leave wood untreated outdoors?

If the timber is naturally durable (see Wood Works No. 2 for information on durability) then it can be left untreated and will ‘weather’ to an attractive silver-grey colour. However, the weathering may be uneven depending on exposure and is usually accompanied in time by some surface fibre breakdown, minor fissuring, and timber movement.

oak cladding weatherd to a silver-grey colour
Silver-grey hue of weathered oak cladding

Though this may be acceptable (or even desirable) in certain situations, if you wish to retain the natural appearance of the timber, apply a colour or reduce the amount of movement in service and extend its service life, then some form of protective treatment is required.

What wood finishes are suitable for exterior use?

failure of surface finish in window joinery
Site applied varnish finish to window joinery resulted in its breakdown on horizontal surfaces, dark staining and water uptake through end grain

The main types of finishes are paints, varnishes, and wood stains.

Paint generally refers to opaque coatings, including conventional gloss paints and modern paints that are available in a wide range of colours, some of which still allow the grain profile of the wood to show through if desired. They are increasingly available as water-based products which are easy to apply and clean up afterwards; no more smelling the house out with white spirit!

Varnishes are basically paints without the colour. This means that they do not protect the timber from the sun’s ultra violet rays and require more regular maintenance. If the surface finish breaks down, water penetrates the cracks which can result in discolouration and the development of blue-black fungi and mould growth. As a rule, we would warn against using varnishes externally unless you are prepared to undertake annual maintenance.

Wood stains are available in a range of translucent colours which allow the grain to show through to varying extents depending on the film thickness and pigment. The benefit of using a wood stain is that they are vapour permeable, allowing the timber to ‘breathe’, i.e. they allow excess moisture in the wood to disperse whilst preventing water entry. They are also relatively easy to maintain, generally only requiring a wash or light rub down, and re-application of finish.

Wood stains do have their limitations though. For example, low build stains may not be as good as paints for disguising blemishes, filling gaps and affording protection to joints to minimise movement in window and door joinery. As they are designed to shed water from a water repellant surface, they are less effective on horizontal surfaces, such as window sills, which may then require an extra coat. Also, they are better suited to rough sawn surfaces than smooth, planed ones.

Ultimately, the choice of finish will depend on such factors as the type of wood product concerned and level of protection required, the ease of re-decoration, and degree of exposure.

Tips

wood stain on cedar decking
Effective use of wood stain on cedar decking

In all cases, best results are usually achieved by applying a finish ‘system’, whereby the preparation and top coats are compatible and made by the same manufacturer, i.e. primer, or base coat (which may also contain preservative chemicals), followed by first and top coats. In all cases, it is important to consult and follow manufacturer’s instructions.

Where a factory applied finish is available, such as on window and door joinery, this is generally recommended as opposed to a site applied finish. This enables the finish to be applied evenly to all surfaces under controlled conditions, often resulting in a superior and longer lasting finish.

Consider the location of the items to be finished. A different product will probably be required for a front door in an exposed location compared to one that it set back in a porch.

Good quality wood finishes come at a premium price, but in our experience are generally worth the money as they are easier to apply, perform better and last longer than cheaper aternatives.

Visit the following finish manufacturer’s websites for further product information and advice on wood finishes:

Osmo UK, manufacturers of natural oil and wax-based wood finishes – www.osmouk.com
Sadolin – www.sadolin.co.uk
Sikkens – www.sikkens.com
Owatrol – www.owatroldirect.co.uk

Advice – The Wood Shop Consultancy – www.timberconsultancy.co.uk

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