Why does my external wood work turn silver/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:

a) Those that form a film or layer on the surface of the wood, such as paints, varnishes, and lacquers
b) 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.