A process of treating wood to reduce the tannin staining of coatings subsequently applied includes the steps of providing a solution of a zirconyl compound in a carrier liquid such as water, applying the solution to a wood surface, and, drying the solution. The pore structure of the wood is modified or sealed so that staining of coating compositions applied subsequently over the surface is reduced. The process is particularly beneficial in cases where the coating composition a clear or a light-colored latex paint, especially white. The preferred zirconyl compound is zirconyl acetate. The treating solution may be modified by addition thereto of a lanthanide compound, especially cerium, to provide UV light protection to a substrate and further by one or more metal cations to impart mildewicidal activity to the composition.