Disclosed are hydrophobic finish compositions and cementitious articles made with the hydrophobic finish compositions. In some embodiments, the article is a waterproof gypsum panel surface reinforced with inorganic mineral fibers that face a flexible and hydrophobic cementitious finish possessing beneficial waterproofing properties. These waterproof gypsum panels have many uses, such as, tile backer board in wet or dry areas of buildings, exterior weather barrier panel for use as exterior sheathing, interior wall and ceiling, and roof cover board having water durability and low surface absorption. The flexible and hydrophobic cementitious finish can include fly ash, film-forming polymer, preferably silane compound (e.g., alkyl alkoxysilane), an extended flow time retention agent including either one or more carboxylic acids, salts of carboxylic acids, or mixtures thereof, and other optional additives. Preferably a pre-coated non-woven glass fiber mat is employed to provide the inorganic mineral fibers for the surface reinforcement.