‘Weathered’ Wood and How to Protect Wood From Weathering

The term ‘weathering’ is used to describe the natural process that outdoor wood undergoes if it is left exposed to the elements without protection. The most evident effect of weathering is a change in the colour of the wood from its natural ‘new’ colour to some form of grey; however, the weathering process involves a lot more than merely a change in the colour.

Factors of weathering

Weathered wood is the result of a complex combination of light, moisture, heat, and mechanical forces, causing physical and chemical changes to the surface of the wood. The two principal elements of weathering are sunlight and water.

UV exposure

If wood is left exposed, a relatively rapid photochemical degradation of the wood surface occurs as a result of exposure to ultraviolet light in the sun’s rays. Initially, the wood begins to take on a yellow or brown tone, which then proceeds to grey. During this process, the UV rays damage the polymer bonds within the wood substrate (cellulose, hemicelluloses, and lignin). Lignin is especially prone to UV degradation, and if not protected, the wood cells will become faded and loose, subsequently eroding the wood surface.

Moisture

Weathering is facilitated by rapid changes in the moisture content of wood. Rain or dew absorbs into the exposed surface, and when the relative humidity of the surrounding air increases, water vapour is also absorbed into the wood. When wood takes on moisture, it swells, and as it releases moisture, it shrinks. When the moisture content at the surface is higher than the moisture content deeper within the wood, stresses can occur. These stresses can cause checking (the opening of fissures on the surface of the wood), warping, and cupping.

Physical changes to the wood surface

As well as chemical and colour changes, physical damage occurs on the exposed surface of unprotected wood. The combined action of moisture and UV exposure leads to a decomposition of the wood surface. This breakdown leads to a reduction in the strength of the cell walls near the surface of the wood. Microscopic and macroscopic cracks or checks form on the surface, and as weathering continues, rainwater washes out degraded material leading to further erosion of the wood surface. As the wood surface is made up of different types of tissue which erode at different rates, the surface becomes increasingly uneven, albeit (depending on the conditions) over a long period of time.

Why does wood turn grey?

In warm sunny climates or cooler climates with little rain, exposed wood can weather to a soft, silver-grey colour. This colour change is due to the degraded lignin being washed out of the wood by moisture. The fibres that remain on the wood surface are high in cellulose content, are whitish to grey in colour, and are more resistant to leaching and UV degradation.

Here in the UK, unprotected exposed wood can retain moisture over time due to our damp temperate climate, and fungi are most likely to grow on surfaces that remain wet for long periods.

The most frequently observed fungus, which grows on wood surfaces, is commonly known as mildew. Mildew doesn’t just grow on wood surfaces; it will grow on various organic and inorganic materials as long as it has a supply of water. The presence of a fungus such as mildew on the surface of the wood will cause discolouration and will also cause the wood to attract and retain even more moisture.

Here, it is often the presence of surface growth that is the principal cause of unprotected woodturning grey. Rather than turning a soft silver grey (or driftwood grey as it is often called), unprotected wood in the UK tends to become a darker grey due to the presence of mildew on the surface.

Protection against weathering

The most effective way to stop wood from weathering is to protect it with a coating (also known as a finish) before it is exposed to the elements. The primary function of a coating is to protect the wood from the two main causes of the natural weathering process, moisture, and UV, and to help maintain its appearance.

There are two basic types of coating systems used to protect exterior wood: 1. Those that form a film or layer on the surface of the wood, such as paints, varnishes, and lacquers. 2. Those that penetrate the surface of the wood, including semi-transparent pigmented stains, opaque stains, and water repellents.

The advantage of penetrating stains is that they are easier to maintain as, unlike film-forming finishes, they do not crack, peel or blister even if excessive moisture enters the wood. Penetrating stains will also provide a more natural appearance as they permit much of the wood grain and texture of the wood to show through.

The most natural appearance for exterior wood can be achieved by using a clear coating system. However, clear coatings exposed to the elements require frequent maintenance to retain a satisfactory appearance, since they offer minimal protection against UV light.

It is generally recognised that pigmented stains offer the best protection against weathering, as the pigments restrict or block out UV light. Opaque coatings provide the most effective and long-lasting protection as they block out UV light entirely.

Do naturally durable wood species such as Western Red Cedar and Siberian Larch need protection?

There is a common myth that naturally, durable wood species don’t need to be protected with a coating. Though this may be true in certain climates, this is not the case here in the UK for the reasons already discussed.

The Western Red Cedar Lumber Association, for example, states that “Although cedar is a naturally durable species, leaving it untreated is not recommended because a finish or protective coating will greatly increase its service life.” The WRCLA also says that “Western Red Cedar performs best when supplied factory finished on all six sides.” Numerous reports by wood scientists and experts support these statements, with the consensus being that the application of a coating is necessary to protect the wood from the damaging effects of moisture and UV, to help prevent the wood from weathering and to curb the growth of fungi that produce mould or stains.

The advantage of factory coated wood

Factory coated wood is applied in ideal conditions and eliminates the variables that affect a site applied coating. Weather, temperature, and humidity all affect the quality of the coating application, so keeping the wood clean, dry and out of direct sunlight before the coating is applied has a highly positive effect on the performance of the coating system.

As recommended by wood scientists, boards are factory coated on all six sides. Applying coating to the rear is essential, as even though unseen, it prevents moisture absorption through the back of the boards, ensuring the cladding and trim components maintain dimensional stability.

Silva has operated an in-house factory coating line since 2013. In that time, we have developed an expertise in the art of factory coating exterior wood. Our coatings of choice are made in Canada by Sansin - a leading manufacturer of environmentally friendly wood protection systems. Since 1986, Sansin has focused on creating the best performing, water-borne wood coating systems in the world.

Sansin SDF™ - a pigmented penetrating stain which protects all exterior wood from moisture and UV

SDF’s specially formulated oils and resins penetrate the wood, providing outstanding weather and UV protection. High-quality nano pigments create vibrant colours that last. SDF is available in a wide range of natural, semi-transparent and solid colours.

SDF is highly water-repellent, so it’s ideal for moisture-rich environments. It allows the wood to breathe, helping to prevent the moisture that causes rot from getting trapped in the wood.

Because SDF penetrates the wood, it leaves the grain and texture visible for a more natural look. It won’t crack, peel or blister, so it’s easy to maintain with a single top-up coat.

Sansin Wood Sealer – a virtually invisible penetrating treatment that allows wood to turn silver-grey while suppressing surface growth

Sansin Wood Sealer is a low-VOC water-borne penetrating treatment. Almost invisible once applied, it is designed to reduce moisture absorption and minimise surface growth and discolouration. Over time, the wood will weather uniformly to a beautiful soft, silver-grey tone.

As Wood Sealer contains no pigments, it offers no protection against UV. However, it aggressively repels water while allowing the wood to breathe. Over time, the wood will grey to a natural driftwood tone while retaining its water-repellency to protect against surface growth. Tests show that, on average, one application of Sansin Wood-Sealer reduces water absorptions by 84% and swelling by 72%.

Wood Sealer erodes gently with time making it easy to maintain. No stripping or sanding is necessary, all that is required is a fresh coat of Wood-Sealer.

Sansin Wood Wash - restores weathered wood surfaces to their natural colour

Weathering only affects the part of the wood near the surface, creating a disorderly arrangement of loosely matted fibres.  Sansin Wood Wash is highly effective in removing unsightly weathering, mould, algae, mildew, staining.  This easy-to-use biodegradable cleaner will successfully restore the wood surface to its original natural colour.