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Best way to check for pinholes is to shine a torch at the mirror and have a look from the other side. Beware - even a newish mirror will look a bit scary when you do this. The signs that the coating has tarnished will be large-scale blotching, or even bits of the coating flaxing off.
The dewing sounds a little odd - is the mirror enclosed at one end of a tube? It's unusual in my experience for a primary in an enclosed system to dew at all, but it has happened on nights of extreme humidity when the temperature has dropped sharply - doesn't sound like Brisbane in spring at all! I don't think there's an awful lot you can do, and I'd be surprised if the surface of the mirror made any difference.
cheers,
Andrew.
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