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Message: <blockquote data-quote="etudiant" data-source="post: 3423492" data-attributes="member: 48052">That probably is referring to the gunk that rains down from some trees, stuff that hardens into little lumps on the glass. Leave it for a couple of days in the summer heat and it really bakes onto the glass. Not easily removed after that other than with lots of lens cleaner fluid and even more patience.Fortunately that is not common and most dirt will be just as readily removed after a month as immediately. Cleaning is always a threat to the lens surfaces, so doing it less often is probably the least risky course.</blockquote>