Exterior Wood Finishing Guide Part 4 - Oil Coatings

24 Sep 2019
Adam Downey

Oil coatings bring out the best in timber and nothing beats the beauty of an oiled finish. However, the species of timber can play a big part in the effectiveness of the oil. An oiled softwood for instance will do nothing to enhance its appearance, whereas on oak or teak it will look fantastic. But, and it is a big but: an oiled finish externally will not last very long at all and that's due to its UV protection properties, or rather, a lack of.

Oiled finishes are deep penetrating, microporous, water repellent and provide a flexible coating.

There are hundreds of clear/colourless products sold as exterior oils, varnishes and wood stains on the market today that the manufactures lay claim to provide good UV exterior protection. Having been in this industry now for 30 years we have tested most and all but only one of these can stand up to the elements whilst providing an excellent finish. Osmo‘s UV Protection Oil.

We have spoken to numerous manufacturers on this subject on how and why they claim their clear product will provide the protection against UV light. We are given the same answer every time, “apply 3 to 4 coats to start with and then a new coat every 3 to 6 months.” They are in fact telling the truth, they do provide protection and of course they do have their merits, but this is clearly only short term protection. Consider the maintenance on your house and see how long you keep it up. Colourless exterior coatings generally offer very little UV protection, if any at all, sun-factor 0.
However they can be used externally and look good for years. With garden furniture for instance. It can be left out for the summer and then put away for the winter. All you would need to do is clean the furniture down and then apply another coat of oil as a maintenance coat.

Some people may want the timber to turn a natural silver grey colour, this is where an oiled finish is preferable to a wood stain. An application of oil every few months to keep the water out will allow the timber to fade gradually to a silver grey colour but it is important to understand that keeping the water out is imperative.
If you want your timber to turn silver grey naturally, careful consideration should also be given to the species of timber in use as not all timbers are suitable for this purpose.

The greying of the timber is caused by a loss of natural colouring, (bleaching from the sun) but it's only the surface that's bleached. This bleached surface is dead wood and needs to be removed to reveal the fresh timber underneath, using abrasive paper or a similar product. There are arguments that oxalic acid can remove the old grey surface and other products such as wood reviver's, wood bleachers etc. All these products are based on oxalic acid, but it is our opinion that none of these types of product will revive the surface back to its original colour.
There are now grey/silver coloured wood stains available and give a stunning effect.

The most common question we are asked today is - can we have oak exterior joinery to be left as natural as possible with no colour change? The simple answer to that question is no. It’s not possible and for the reason already explained above.

Now then this leads us on to the Osmo UV protection oil mentioned earlier. Osmo UV protection oil contains special clear ingredients which do not allow UV rays to hit the wood surface with a UV protection factor of 12, effectively preventing the greying process. This is the first clear oil based finish with UV protection for exterior use that works, and it works well. Although a clear oil, applied to oak or any timber it will still change the colour slightly. It will bring out the colours within the oak, producing a more pleasing satin/matt effect. But it won't leave a natural untouched finish.