Exterior Wood Finishing Guide

28 Sep 2022
by Alpha Quad

This exterior wood finishing guide will give you some insights we have developed over the years. We discuss treating exterior wood, wood stains, solvent vs water based wood stains, oil coatings, wood varnishes and references, and exterior wood maintenance.

Treating exterior wood

Wood that is subject to outdoor exposure will eventually rot and decay from environmental factors such as water absorption, humidity, heat, UV light, pollutants, bacteria and fungi. Treating exterior wood surfaces is essential to protect and preserve your eternal timber.

The UV light (ultraviolet) from the sun is the main culprit causing the timber surface to deteriorate. This breakdown, depending on the level of exposure to the elements and the species of wood, can take as little as one week to start to show signs of degradation. Once exposed to the elements without protection, timber will develop small surface cracks and 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 timber's moisture 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 note is: do not allow untreated timbers to be exposed to the elements, especially during the winter months or when rain is expected. Timbers with high tannin content, Oak and Idigbo, for example, will create problems if they are exposed to moisture. Treating exterior wood surfaces is essential ahead of the winter months.

As part of your exterior wood finishing and to stop this degradation, a protective coating must 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, developed and tested by the manufacturers to give long-term protection. If only everything were 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 attacks on the timber surface and should be used on all softwoods and some hardwoods.

The protective coating must also incorporate water repellents, UV absorbers, and translucent pigments. It 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 consider:

The timber species you use will determine how long a surface coating will perform and last with your exterior wood finishing. Oak and teak, for instance, will require more maintenance than mahogany over the same period.

South facing elevations are subject to the sun's effects for longer, and during the summer months, the surface becomes very hot and will break down far quicker than north, east or west elevations. A south-facing elevation can break down 75% faster!

Another consideration when treating exterior wood is the damage caused by wind, which has the effect of sandblasting exposed timbers and quickly removing the coating. Oak in a south-facing exposed elevation will start to break down within six months,

So it’s worth considering your choice of timber in these areas.

Exterior Wood Stain

Wood stains contain pigments to provide colour while accentuating the natural grain of the timber as well as providing protection against UV damage. They are microporous, and some wood stains contain active biocides to prevent mould growth in or on the finish itself. However, this is no substitute for a wood preservative, which focuses on doing this job best! Preservatives can be used in combination with an exterior wood stain.

When it comes to exterior wood finishing, think of a wood stain to timber as sun cream to us; if you sit out in the full sun, you will burn, so you apply a sun-cream/blocker with a certain sun factor rating for protection. The higher the rating, the longer and better the protection. Wood stains work to the same principles. 

The colour of the exterior wood stain also affects the protection factor. The lighter the colour, the lower the protection. The darker the colour, the higher the protection. Then we get to the point where the darker the stain, the more heat it absorbs, causing the timber to swell (depending on the species) and doors and windows to become difficult to open and close.

Colour pigments within the product filter out the harmful UV light, so generally, if there are no pigments, you have no protection causing the coating to lose its sheen. It becomes brittle, cracking occurs and ultimately, the coating fails.

Good quality pigments are expensive, so a quality product is more costly, but you get what you pay for, and better quality pigments hold their colour for longer!

Manufacturers produce exterior wood stains in various colours, including colourless, which usually will not give any long-term UV protection. Most manufacturers have now removed them from sale. However, colourless does have its merits; for instance, treating exterior joinery before it leaves the joinery workshop. Stage one exterior wood finishing before you even get the timber! Timber is then protected from the weather for a short period, enabling the joinery to be secured in place, and then a coloured base coat is applied, followed by the top coat or a coloured top coat only providing a lighter shade. Colourless top coats can also be applied to a fully finished system to add further protection without darkening the stain further.

Colourless exterior wood stain top and base coats should not be used together as a finish on their own.

There are also different levels of exterior wood stain build (meaning the film thickness when dry), low build, medium build and high build. The number of coats to apply (as instructed by the manufacturer) ensures that each coat lays down the correct material thickness to ensure full system performance. The build from the coating is designed for a particular application.

Base coats are low build, and have lower viscosity, easily penetrating the surface, sealing it and providing a matt finished base for the top coat.

Top coats are medium to high build, with higher viscosity, providing a weather-protective satin finish.

High build systems tend to be paint and varnishes and gloss finishes. You may also come across some high-build wood stains although this type of product will provide greater UV protection, it would be so heavily pigmented it would obliterate the grain of the timber and provide a paint type of finish.

Applying the wrong exterior wood stain build of product to a substrate will give inadequate protection, also making a product go further (known as scrub out) will also produce a lower build than required.

External Wood Stains - Solvent vs Water Based?

Water-based or solvent-based external wood stains?

Choosing between water and solvent-based wood stains for your exterior wood finishing requires careful consideration of factors such as the type of wood, condition of materials, internal or external use, weather conditions at the time of application and health and safety.

Both water and solvent-based external wood stains can be used on internal and external timber.

A water-based wood stain is easy to apply and is quick-drying, has low VOC and low odour.

When using low VOC wood stains, projects can be completed in one day. Water-based products do not contain any active ingredients, so a water-based wood preservative (such as Impra Prevent or Osmo WR Base Coat should be used with this product.)

Compared to solvent based wood stains, water-based wood stains may have a shorter wet edge time, so careful consideration is needed when applying to a large surface area and when the weather is very warm. Solvent-based wood stains provide better flow, have a longer wet edge time and are least susceptible to weather conditions, so more flexibility is available to the user when applying.

In our tests on hard and soft woods using Impra profilan (water-based) external wood stains and spirit-based external wood stains (Impra and other well-known brands) on exterior exposed surfaces, the Impra profilan wood stain lasted five years before a refresher coat was required. This was two years longer than the Impra spirit based wood stain and 3 to 4 years longer than the others!

Quality water-based wood stains for your exterior wood finishing are generally more expensive than spirit-based wood stains, the technology required to produce a top-quality product requires a great deal of research and development, and most manufacturers do not have the resources. Water chemistry has made great advancements while solvents have remained unchanged. As a result, water-based wood stains today are equal to or superior to solvent-based wood stains. Unfortunately, most water-based wood stains in the UK today perform very poorly and can only be classed as coloured water.

Our recommendation would be the Impra Profilan after testing the product multiple times. It is also recommended in several other independent tests, including a Which? best buy award for the Opac Paint!

Oil for Exterior Wood

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 for exterior wood. 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 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 oil for exterior wood, varnishes and wood stains on the market today that the manufacturers claim to provide good UV exterior wood finishing protection. Having been in this industry now for over 35 years, we have tested most, and 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 of oil for exterior wood on how and why they claim their clear products will provide 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 telling the truth, they do provide protection, and of course, they do have their merits, but this is 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… 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, which 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 revivers, wood bleachers etc. All these products are based on oxalic acid, but it is our opinion that none of these types of products will revive the surface back to its original colour.

There are now grey/silver-coloured wood stains available and give a stunning effect for exterior wood finishing.

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

Now then, this leads us on to the Osmo UV protection oil for exterior wood 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.

Exterior Wood Varnish & References

An exterior wood varnish provides a hard, transparent film coating and is usually only available in a full clear gloss finish. Varnishes are not microporous and are not flexible, and this inflexibility can cause problems when the timber moves. Small cracks can develop in the varnished coating allowing moisture to penetrate the surface. Exterior wood finishing varnishes now contain UV absorbers; this extends the lifetime of the coating and helps to keep its glossy finish. It's difficult to say how long a varnished coating will last, but depending on its directional aspect, it can be 2-3 years before some maintenance is required.

Exterior Wood Maintenance

Once you’ve taken the time, and expense to enhance, protect and preserve your timber, exterior wood maintenance will protect this investment and save money in the long run.

The secret to low maintenance of exterior wood coatings is to monitor your exterior joinery, keep it clean and remove any moss, algae or other contaminants that settle on the coating.

Exterior wood finishing stains erode over a period of time, and colour loss is gradual. Maintenance will usually involve (after cleaning with soap and water) an application of a refresher top coat. A good quality wood stain should last about three years in a south facing aspect, and longer in other directions.

Oiled finishes generally will not last as long as a stained finish, again depending on its exposed directional aspect. But close monitoring and a refresher coat are all that are needed when necessary.

Varnished coatings tend to become brittle and require the top coat to be removed by sanding and then a refresher top coat applied.

The consequences of neglected exterior wood maintenance will undoubtedly cause the entire coating (top and base coat) to erode, exposing the timber to the elements. Once exposed, moisture will penetrate the timber creating an unstable wet surface. Maintenance will then require the removal of all of the existing coating back to bare timber, sanding the surface again and then reapplying the base and top coat. Any attempt to spot treat any area will result in a very patchy appearance and lack of uniformity and will impair your exterior wood finishing.