An orthodontic appliance can include a shell having a plurality of cavities shaped to receive a patient's teeth is disclosed. The orthodontic appliance may comprise an interior surface and an exterior surface, and an elastic coating covering at least a portion of one or more of the interior surface or exterior surface of the shell. A stiffness of a portion of the orthodontic appliance corresponding to the portion of the shell covered by the elastic coating can be determined by a stiffness of the elastic coating.