The mirrors have performed well in our saltwater environment. We have a number of hermetically sealed mirrors from Japan and Germany under long-term degradation tests. It appears that a small amount of UV transmits through to the adhesive causing eventual failure with attendant capillary water infiltration. Georgia Tech tried to protect the gray primer paint with lacquer paints which caused the silver to rapidly delaminate from mechanical stress. I have since stopped using substrates and will not add additional coatings to the gray painted mirrors.
Substrates cost more than glass which is why tall office buildings are mostly covered in glass.
Mirror manufacturing is more of an art than a science. More than 20 years ago SERI was investigating the chemistry of the mirror industries and encountered many trade secrets, but did find a curious alchemy of process steps. Sugar was one of the ingredients and nobody could explain why, it was just in the recipe. The research on glass mirror was terminated during the Reagan Administration. He responded with some interesting ideas.