Tannin is a dark brown, acrid oil found in many trees, including oak and cedar. In outdoor use, tannin will become concentrated, staining and darkening the wood’s surface, especially following the grain.
When choosing a wood protectant for outdoor cedar decks, fences, and furniture, tannin bleed through is a major factor to consider. Recently a manufacturer of pre-finished decking materials requested a head-to-head test of the wood sealer they currently use versus Hemp Shield.
In the ASTM test shown here, cedar 2X4’s are masked, the sealers are applied to the test areas, and each board is exposed to a 500 hour torture test of heat, humidity, and intense ultra-violet (UV) light. The masked edges show the test stock’s original appearance.
The national brand samples are darkened and discolored by tannin from below, with the surface finish almost completely removed by UV and humidity from above. In contrast, the Hemp Shield samples are nearly unmarked, with only a few visible stains. The Hemp Shield finish has preserved the original tone and color of the wood, while the boards treated with Miller have been visibly greyed where tannin has not completely discolored them