Most people know that wood is comprised of cellulose, hemicellulose, and lignin.  There is one more minor component that is generally classified as “extractives.”  These are largely phenolic-based compounds that can be extracted from the wood using various means.  Some extractives can be removed by water.  An unprotected stick of exterior wood will soon begin to develop an extractive gradient as the water-soluble compounds begin to move by a diffusion gradient to the surface.  This gradient is similar to what develops in service with creosote-treated utility poles.   This also occurs with interior wood but much slower.  As the extractives move to the surface, the surface becomes darker in color and also is less “active.”  A clean wood-finish bond cannot be achieved, and early finish failure is likely.  This is particularly true for woods that are known to be high in extractives such as oak and hickory.