Wood that is subject to outdoor exposure will eventually rot and decay from environmental factors such water absorption, humidity, heat, UV light and pollutants.
The UV light (ultraviolet) from the sun is the main culprit causing the timber surface to deteriorate. This break down, depending on the level of exposure to the elements and the species of wood can take as little as 1 week to start to show signs of degradation. Once exposed to the elements without protection, timber will develop small surface cracks as well as a loss of colour. The grain will become raised, the surface will become roughened and the timber will absorb moisture, the process of wood rot has started.
It is worth mentioning at this point that the moisture of the timber is extremely important. Once the timber has taken on moisture it may be difficult to successfully apply an oil based product onto the surface as oil and water do not mix; therefore the product will not penetrate the surface and will simply sit there and eventually wash off.
The point to bear in mind is: do not allow untreated timbers to be exposed to the elements especially during the winter months or when rain is expected. Timbers which have a high tannin content, Oak and Idigbo to name two will create great problems if they are exposed to moisture.
To stop this degradation a protective coating needs to be applied to the surface of the timber. The protective coating can be in the form of a paint, wood stain or oil and each type of coating has to be applied in a specific way, which has developed and tested by the manufactures to give long term protection. Only if everything was as simple as that!
On certain species of timber it is always advisable to apply a wood preservative to the bare timber before you apply a protective coating, these preservatives are formulated to prevent fungal attack on the timber surface and should be used on all softwoods and some hardwoods.
The protective coating must also incorporate water repellents, UV absorbers, translucent pigments, must be fade resistant, stay flexible and give long term protection and of course needs to be easy to apply.
A couple of last points to take into consideration:
The timber species you are using will determine how long a surface coating will perform and last. Oak and teak for instance will require more maintenance than mahogany over the same period of time.
South facing elevations are opened to the suns effects for longer and during the summer months the surface becomes very hot and will break down far quicker than a north, east or west elevation. It is possible for a south facing elevation to break down 75% quicker!
Another consideration is the damage caused by wind, which has the effect of sandblasting exposed timbers and will quickly remove the coating. Oak in a south facing exposed elevation will start to break down within 6 months,
So think on about your choice of timber in these areas.