Solid proppants are coated in a process that includes the steps of: (a) coating free-flowing proppant solids with a first component of either a polyol or an isocyanate in mixer; (b) adding a second component of either an isocyanate or a polyol that is different from the first component at a controlled rate or volume sufficient to form a polyurethane coating on the proppant solids; and (c) adding water at a rate and volume sufficient to retain the free-flowing characteristics of the proppant solids.