Naturally occurring tannin compounds in all species of wood are in a large part responsible for its beauty, stability and durability. However, sometimes these water soluble tannin compounds migrate to the wood's surface. A high concentration of these compounds may cause a discoloration referred to as tannin bleeding or stains, and can turn the wood very dark if unprotected and exposed to moisture. Tannin compounds may bleed around pressure points such as nail holes, strapping marks or where water is able to penetrate the surface, especially wherever condensation forms. Another example of tannin stains which may become permanent, are the black marks left from pine needles, leaves and other debris which are allowed to accumulate on the deck.