These stains are the result of naturally occurring chemicals in wood which migrate to the surface and are often called bleeding. The resulting stain can discolor the surface of the wood, and may create permanent discoloration below the surface. They provide different species of wood i.e., redwood, cedar, pine, oak, etc. many of their special characteristics and aesthetic appeal. These stains are carried to the surface by high humidity, fog, rain or the natural ingredients contained within the wood. Extractive stains that occur are of two types, those that result from movement of water soluble materials and those requiring solvents for movement. The most common discolorations are naturally occurring pigmented compounds. These stains are most pronounced in darker woods such as cedar and redwood, but are also frequent in pine, douglas fir and similar lighter toned woods. They especially develop over time on plywood painted with water-based products.