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Post script; whilst the U.S coaters I've contacted won't touch overcoated silver because (they say) ,it is still permeable on the molecular level, the boss of Optical Coatings Associates (David) has told me that his overcoating has the problem licked because it's non porous. It would be interesting to know how long they last in practice. But David has been helpful, even offering to send me a coated sample to play with. I'm inclined to stay conservative, especially since over here in NZ I regularly get dew on the primary! I thought that was a near impossibility!
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