A.Tom Brown responds: Exposure to sunlight will cause most species of wood to darken, but the darkening is not caused exclusively by UV radiation. Color changes occur when the pigments within the wood oxidize. This oxidation is caused not only by the "free radicals" generated by UV radiation, but also by the presence of oxygen. The UV absorbers found in many finishes inhibit or partially block UV radiation from reaching the wood, but since no finish is totally impermeable to oxygen, the color change will still occur, though more gradually.
To postpone the inevitable, I recommend using a film-forming finish, which blocks oxygen penetration much more effectively than a penetrating finish. Make sure the finish contains a UV "absorber," not a UV "inhibitor." UV inhibitors are used to protect the finish film from UV degradation, and do not protect the wood pigment...
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