Finishing Exterior Wood

The appearance of wood can be modified with the application of an architectural coating. Architectural coatings are surface coverings such as paint and stain applied to a building or exterior structures such as a deck. Coatings are multi-functional, not only being decorative, reducing the effort needed to clean buildings and structures, but also providing protection against moisture uptake and assisting in extending the life of wood. However, coatings cannot be considered as substitutes for preservative treatment. On this page, we explain the basics of different types of exterior wood coatings, and what they can and can’t do for wood.

Types of Coatings – Opacity
Architectural coatings available for wood generally include paints, stains, varnishes and water repellents. There are a number of ways to classify coatings, and it can be confusing to understand the differences between types of products.  One common method is to differentiate based on appearance: coatings are often identified as 1) Opaque; 2) semi-transparent or 3) transparent.  These terms indicate how much of the natural wood features will still show through the finish. Opaque coatings include paints and solid colour stains.

An opaque coating doesn’t allow any of the wood’s natural colour to show through and may also hide much of its texture. It thoroughly protects the wood from damage caused by sunlight. It can also help keep moisture out of the wood.  These coatings tend to last the longest.

transparent or semi-transparent finish such as a stain or water repellent may change the colour of the wood, but because it exposes the grain and texture, the wood still looks “natural.”  These finishes help keep moisture out of the wood but there is considerable variation between stains in their ability to restrict moisture ingress. They also help protect the wood from sunlight damage to varying degrees depending on their content of organic UV absorbers or inorganic pigments. The difference between transparent and semi-transparent coatings is also sometimes unclear.  Transparent coatings allow more grain and texture to show through. Transparent exterior coatings labeled as “clear” may still contain some pigment to enhance wood’s natural colour and assist in revealing the wet edge during application. However, please note that clear products intended for interior use only are not appropriate for exterior use, as they will quickly fail in sunlight.

There are many transparent products marketed as providing water protection for wood (water repellents) – these might technically be considered wood “treatments” rather than wood coatings as they mainly provide water protection and help reduce checking (splitting), and have very limited, if any, UV protection.  This means they have shorter lives than pigmented finishes, but they do help slow down the weathering process by restricting water ingress.  Please note that water repellents are often solvent-borne and contain wax which affects the adhesion of subsequent coatings, which means most of these products should not be used as a pre-treatment beneath paint.  However, transparent water repellents have the unique benefit of being the most aesthetically-forgiving treatment when there is lack of maintenance.  In other words, these products don’t change the colour of the wood, so bare patches of wood aren’t evident as the product wears away.

Types of Coatings – Carriers
Another popular way to categorize coatings is by the type of carrier (the base) – products are water-borne or solvent-borne.  When low VOCs (volatile organic compounds, which are implicated in lowering air quality) and easy clean-up are important, a water-borne product is usually the choice.  Water-borne coatings now dominate the market due to increasing environmental regulatory requirements and customer demand.  Compared to solvent-borne finishes, water-borne finishes usually have less odour and can be cleaned up with water instead of mineral spirits. Water-borne coatings are generally more flexible (less prone to crack as wood shrinks and swells from moisture changes) and more vapour permeable. Water-borne coatings, particularly acrylics, generally fade and chalk much less than alkyds. The technology for water-borne finishes is now mature, and they can match or exceed the properties of solvent-borne products.

Water-borne paints are often called latex. Solvent-borne paints are commonly known as oil paints.  Also, paints labeled as alkyds are typically solvent-borne (but not always).  Although it is popular to refer to paints as either latex or oil/alkyd, it is more useful to think of them as water-borne versus solvent-borne.

Types of Coatings – Film Thickness
Sometimes wood coatings are classified by the thickness of film they form on the surface of the wood.  Paints, solid colour stains, and varnishes are often called film-formers, as these create a layer of material sitting on top of the wood.  Semi-transparent stains, transparent stains, water repellents and natural oils are often thought of as penetrating the wood rather than leaving a thick film on top of the wood. Hence, they are often called penetrating finishes.  However, all coatings leave a film on the surface – thick for some, thin for others – and the “penetrating” products only penetrate a very small distance into the wood.  Nonetheless, it’s helpful to know if a product leaves a thick film, as this type of product can be more difficult to remove if allowed to degrade prior to refinishing.  This is because their failure modes are different – a thick coherent coating like paint fails by cracking and peeling, whereas a thin-film “penetrating” product such as a stain fails by erosion.

Can Coatings Protect Wood?
Coatings protect wood, but coatings do not actively protect against decay.  Their purpose is primarily aesthetic. But they slow down the damaging effects of weathering, and do provide some moisture protection, which is a decay factor.  Coatings also help preserve the natural durability of species like western red cedar, by helping to prevent the natural protective agents in this wood from washing out.  The protective benefits of all coatings are, of course, dependent on proper maintenance of the coating.  No coating will last indefinitely, and all need to be periodically reapplied.

Weathering
Weathering is the slow surface degradation that occurs when wood is exposed to the weather. Note that weathering should not be confused with decay (rot) caused by decay fungi, which can penetrate deeply into wood and significantly reduce wood strength in a relatively short period.  In contrast, weathering of wood is caused by UV, water, oxygen, visible light, heat, windblown particulate matter, atmospheric pollutants, sometimes together with some specialized micro-organisms.  Under these factors, wood exposed outdoors above-ground with no coating will quickly change appearance. The colour will change due to the photodegradation, chemical leaching and other chemical reactions; light woods will typically darken slightly and dark woods will lighten, but all woods eventually end up a silvery-grey colour.  The surface will also roughen, check and erode, due to repeated ultraviolet radiation, wetting and drying, and mechanical abrasion from wind-blown particles. Hence the weathered wood will have a “rustic” look.  Some microorganisms and lichens may colonize wood, but the wood’s surface condition does not usually favor decay.  Note that weathering only occurs on the surface of wood, usually at a depth of 0.05 to 0.5 mm.  As long as decay doesn’t start, weathered wood will still be completely serviceable for years.  In order to reduce weathering and improve the aesthetic appearance of wood, wood exposed outdoors above-ground can be protected with coatings.

Link to articles on weathering at the website of USDA FPL:
Weathering and Protection of Wood:

http://www.fpl.fs.fed.us/documnts/pdf1983/feist83a.pdf

Weathering of Wood:

http://www.fpl.fs.fed.us/documnts/pdf2005/fpl_2005_williams001.pdf

Acknowledgements

The material was reviewed by Dr. Sam Williams of the US Forest Products Laboratory, Dr. Philip Evans of the University of British Columbia, and Mr. Greg Monaghan, a Specialty Coatings Group Leader at Rohm and Haas, but the final content does not necessarily reflect their views on all points.