The aluminium coating is prevented from oxidising by the application of a protective silicon oxide coating. It keeps the reflectivity at a very high percentage over many years. It is not a perfect seal, and the aluminium coating does slowly degrade.
Professional instruments, such as the 4m AAO puppy, is just the naked aluminium. As a result, the coat of Al degrades over a couple of years, and needs to be recoated. This is done with facilities built into the observatory.
There is a really good set of before and after series of photos about the recoating of the 200" Hale Telescope. The recoating of this 5m mirror is also done every two years. The difference in the appearance before and after is quite striking:
http://www.pfonline.com/articles/mirror-mirror
The 4m AAO instrument, and its smaller siblings also undergo the same recoating regime.
Now, recoating OUR mirrors is more problematic. Non-overcoated mirrors are have a sodium hydroxide solution put over them to remove the aluminium. The Pyrex glass is cleaned, and the aluminium is reapplied.
Silicon oxide overcoated mirrors need to have this protective coating removed. Problem here is the substrate, the parent glass the mirror is made of, NEEDS to be Pyrex, or SuperMax, not BK4 or BK7 glass types, and certainly not plate glass. The chemicals used to remove the overcoat will also dissolve these glass types. In these situations the overcoating needs to be mechanically removed by grinding/polishing. Recoating overcoated mirrors isn't cheap, and more so those using chemically sensitive substrates.
Alex.