A damaged AR coating cannot be repaired. However, the coating can be removed using a stripping agent. If the damage did not severely compromise the underlying hard coat or lens material then the AR coating can be reapplied. If the underlying hard coat is damaged then this to can be reapplied and then the AR coat reapplied . The hard coat is the base upon which the AR coat is deposited. If the damage has affected the lens material itself then there is no other option than to replace the entire lens. I am an optician who has been in practice for 18 years and who runs an optical lab manufacturing optical lenses with hard coatings and AR coatings.