The invention is generally related to a method for producing, in a consistent manner, contact lenses each having an intact durable coating thereon, wherein the coating is a hydrogel coating formed by covalently attached a hydrophilic polymeric material having azetidinium groups onto a base coating of a polyanionic polymer on a contact lens at a relatively low temperature (e.g., from about 40° C. to about 60° C.). The coating temperature for forming a durable hydrogel coating on top of the base coating of a contact lens can be significantly lowered by raising, in situ, the pH (to about 9.5 or higher) of a reactive coating solution which contains the hydrophilic polymeric material having azetidinium groups.