Tannin is the natural oil found in cedar that gives it outstanding resistance to fungal decay. It is rare in white cedar to ever see it appear, as it typically remains in the wood. It is a water-soluble substance. However, tannin can appear in rare cases following intense periods of heavy rains and intense sun. Tannin in white cedar will not stain the surface and will usually weather away within 3 to 6 months.