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One thing that I have not seen mentioned in this thread is that why Isopropyl Alcohol is so good as a solvent cleaner is it's rate of evaporation. Ethanol or Acetone will condense any contaminants in the local atmosphere onto your optic surface due to their high rate of evaporation at normal room temperature.
Isopropyl Alcohol evaporates at a rate where it forms a 'protective' cloud of solvent vapour so no contaminants can condense or settle on your optic surface.
All antireflection coatings are porous at the molecular level so will incorporate contaminants. Some are better than others. It depends on the vacuum deposition method.
A plain Al coating will form oxide within seconds of exposing to air (oxygen). A 100nm Aluminium Oxide layer will form after a couple of days. The reflectivity will drop from 90%+ to about 84% or less over time. As an over coating for Al, SiO2 is deposited as SiO and this converts to SiO2 over time.
So NEVER clean a freshly coated mirror until many weeks after coating.
Pinholes in coatings are the main reason for deterioration as contamination eats away at the glass coating interface.
I spent many happy years producing vacuum coatings at Kodak in the Reseach Lab so I most probably have forgotten more than I know now.
Bert
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