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As others have said, coaters don't rework the glass. They use a very strong corrosive solution to dissolve and remove the aluminium coating. Clean the glass first chemically then some follow up with an ion beam clean. I knew one aluminiser in the 1980's in Brisbane who used aqua regia, a mix of three parts concentrated hydrochloric acid to one part concentrated nitric acid. I don't know what Wayne Sainty at Palmway uses. The overcoating sits on top of the aluminium so when the aluminium dissolves, the silicon monoxide overcoat releases. It is chemically very similar to glass and neither the glass nor the overcoat react or are affected by the corrosive used.
Five years ago, I re-coated a 6" mirror that I hand made in the 1970's. The optical figure is excellent so just buying an off the shelf replacement was pointless. I had Palmway re-coat with an enhanced reflectivity aluminium coating with a silicon monoxide overcoat. The cost in late 2018 was $420 inc return freight. I paid the freight to them. Because I made the mirror and the quality of the optics, it was worth re-coating to me. An entire 6" commercial mirror would have cost much less that the re-coat but that wasn't the point.
I don't know how much a 12" mirror would cost to recoat, it would be at least $420 plus inflation plus some extra for the bigger mirror. Wayne is a nice guy and very helpful. Just contact him for a quote.
Joe
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