Abstract: A method of increasing water repellency of an organic porous substrate, such as wood, is provided. The method comprises applying a dispersion of inorganic nanoparticles in a liquid carrier onto a surface of the substrate. The liquid carrier is selected to evaporate substantially entirely at room temperature. For example, the liquid carrier may be selected so that at least 95% of the liquid carrier will evaporate at room temperature, preferably in a 24 hour period following application to the substrate surface.