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Isopropyl is a standard for removing greasy deposits on aluminium /overcoated coatings and antireflective thin film coatings. I've found the usual routine of cleaning first with wads of cotton wool and warm dilute water before distilled rinse, isn't enough to get rid of a very tenacious greasy layer on mirrors cuased my air pollution and Eucalyptus oil if you live near the bush, so isopropyl is needed.
The standard test for cleanliness of an aluminium coating is to breathe on it and look out the breath pattern . If it is featurless and even then the coating is clean. if it is spotty and blotchy with big dark spheres then you have an oily deposit. I find that with a clean mirror displaying some greasy deposit even after a standard clean and isoproply , that a buffing with cotton wool and just the moisture of solid breath gives enough action to lift it. There is no danger of scratching the overcoat ( which is as hard as glass) so long as your materials are clean, and only a contamination that is Silica like will be hard enough to cause scratches.
I disagree with one poster that many modern optics are not over-coated. Unprotected aluminum surfaces will look noticeably dull after 6 months of oxidisation and do not cut it in the commercial world. I would say that uncoated surfaces these days would be extremely rare, although there is one aluminisation service in Australia that does not do it.
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