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Question: I make small boxes and enjoy using colorful exotic woods such as purpleheart. I just hate the way it turns brown over time. What is the best finish for this type of wood if I want it to retain its rich purple hue?

Answer: Purpleheart, also known as amaranth or violet wood, is renowned for its chameleon-like quality. The heartwood is initially a muddy gray-brown, but when it is cut and exposed to the air, it changes to a strikingly bright, vibrant purple. Prolonged exposure to air and light then darkens the wood to a dark-purplish brown or dark brown, but the purple color can be restored by sanding or re-cutting the wood.

Uncut purpleheart Variations in purpleheart Darkened purpleheart

Short of an opaque pigmented coating that completely obscures the wood and blocks UV light, no finish can completely keep purpleheart, padauk, redheart and other brightly-colored woods from darkening over time . However, a clear finish that contains UV absorbers can at least slow the process by providing some protection from air and light. UV absorbers are chemicals added to clear finishes that work like sunscreen and absorb UV radiation, dissipating it as heat. However, as with sunscreen, the UV absorbers wear out over time, so the protection is only temporary.

The most effective UV-resistant coatings are marine-type finishes, such as McCloskey Spar Marine Varnish and Waterlox Original Marine Finish . Marine finishes tend to have more UV absorbers and can better withstand direct sun than traditional spar varnishes. Lower-end home center finishes marketed for their UV resistance very often don't contain sufficient UV absorbers to be effective. Many oil finishes, regardless of the amount of UV absorbers they may contain, are applied too thinly to provide any real protection. These finishes don't build on the wood, so there isn't adequate thickness for the UV absorbers to be of much use, nor do these thinly-applied finishes effectively keep air away from the wood. For best results, a thick film must be built on the wood.

For continued UV resistance, finishes must also be properly maintained. This entails sanding off the top coat every year or so or when the finish dulls, and re-applying a couple of coats. Projects or pieces that remain indoors with minimal exposure to sunlight will not require such frequent maintenance.

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