Dave- You are one homemade telescope mirror ahead of me. I subscribed to this list in the hopes of learning enough to get started. When it comes to big heat formed glass blanks and aluminization I have a lot of experience. My blanks are 50" in diameter and only 3/8" thick. I do not grind the blanks. I have a 90" diameter vacuum tank that I use to aluminize the blanks.
There is always a relection at the interface between materials with different indexes of refraction. For example, the interface between air and a piece of ordinary window glass produces something like a 3% - 4% reflection. Lens typically have an anti-reflection coating to reduce these reflections. In the case of mirrors 1/2 wave protective coatings ensure that the light reflected from the protective coating is "in phase" with the light reflected from the aluminum. This is analogous to the concept to setting antenna lengths to multiples of 1/4, 3/4, .. wavelengths to prevent reflections. For mirrors we want to maximize the constructive interference. Obtaining a uniform 1/2 wave thickness protective coating is quite difficult using the "in vacuum" techniques typically employed for SiO, MgF, and even Al2O3 coatings. The method I use to protect my mirrors produces a nearly perfect uniform thickness Al2O3 and it is as easy as falling off a log.
-- Stan Thomas <thomas@cosmic.physics.utah.edu>