An existing metal structure can be reinforced by providing a reinforcing metal layer on the metal structure in spaced apart relation to thereby form at least one cavity between inner surfaces of the metal structure and the reinforcing metal layer. An intermediate layer including uncured plastics or polymer material is injected into the cavity, and the plastics or polymer material is cured to adhere to the inner surfaces of the metal structure and the reinforcing metal layer. During the injecting and curing process, the reinforcing metal layer is constrained by at least one restraint member that is held to the existing metal structure by at least one magnetic clamp.