An improved primer formulation for composite building materials, such as materials that are generally cementitious, gypsum, or of another inorganic building material, such as those containing cellulose, glass, steel or polymeric fibers. The improved formulation effectively blocks moisture from penetrating the composite building material and is better than alternate or conventional primers. The formulation also improves adhesion and prevents peel failure of a topcoat when applied to the composite building material. The improved formulation acts as a weather-guard and a hydrophobic treatment to all surfaces of the composite building material upon application.