Abstract: Oriented strand board is post-fabrication treated with a polymeric coating which includes isocyanate constituents to produce a construction material which is smooth, has high gloss, and which is resistant to water, vapor, and chemical penetration. The isocyanate constituents are most preferably MDI or PMDI, and may be present in a coating composition alone or in combination with other binder materials such as PEEG or other polyols. Prior to application of the coating composition, the surface of the oriented strand board is pretreated using heat and pressure to compress and densify wood at the surface, and to smooth out minor surface irregularities. The coating composition is then applied to the wood surface, and is cured to form a polymeric coating which bonds to the wood material.