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If you breathe on your mirror and you get a blotchy pattern, with dark spheres of all various sizes, it has oil deposit on it which is very tenacius..I often used methylated spirits first after a rinse to help cut the greasy film . My scope is stored in a shed and doesn't have access to the primary for a close fitting cover .
When the breathe pattern is smooth then the surface is oil free - this is how coaters test for a clean surface putting in the chamber .
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