Methods are disclosed for treating zircon-containing forming structures, e.g., zircon isopipes, with one or more treatment glass compositions in which defect-causing reactions between the zircon of the forming structure and molten glass are suppressed at the delivery temperature of the treatment glass. The treatment compositions can be used during start-up of a forming structure, between runs of the same production glass on a given forming structure, and/or when transitioning between runs of two production glasses on a given forming structure. The treatment compositions can be used with production glasses that are ion-exchangeable.