Cause
Darkening of wood is caused by ultra-violet radiation. This can take place on wood that has never been coated with a protective coating. It also can occur under clear varnishes and lightly pigmented stains. Different degrees of darkening can occur on different woods. If moisture penetrates into the wood, varnish coatings will peel due to wood decomposition under the coating. Woods that are exposed to weather are not protected against wood-destroying organisms. Eventually the wood ceases to be a coatable surface.

Solution
New woods must not be exposed to harmful sun rays for extended time periods. Paint and stain adhesion performance is enhanced when the new wood is painted or stained immediately after installation. Wood that has been allowed to darken (decompose) must be sanded to fresh wood. Wood that cannot be sanded to fresh wood must be replaced. Coatings can be applied to treated woods after the moisture content has dropped to a paintable level. Semi-transparent stains must be used on all horizontal (walk-on) surfaces. Paints, semi-transparent and solid color stains can be used on vertical surfaces.

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