The availability of a large variety of wood types with a wide range of properties and color shades offers a suitable material for almost every application. The possible applications of wood are determined by two factors: its mechanical properties and the composition of ingredients. Wood contains 25 to 30% lignin, which fills the cell walls between cellulose components and bonds them together. While lignin itself is insoluble in water, UV radiation separates it into water-soluble components that can be washed out when the wood moisture content changes. This makes the upper layers of cellulose fiber lose their cohesion, so they can no longer provide a suitable substrate for coatings. As a result, large pieces of varnish and coating are detached. As air penetrates the cracks between the cellulose fibers, the reflection of light at the borders makes the damaged coating appear cloudy and milky.
Completely weathered wood surfaces look silver-gray, which is the original color of cellulose. Depending on the type of wood, UV radiation can penetrate up to 2 mm into the wood surface.2