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Normally the newer coatings on filters can handle a reasonable cleaning without damage. The Isopropyl spray and clean paper towels usually does a good job.
I got a set of UBVRI filters from the States recently and the front surface was VERY gunky... looked like a mixture of vaselene and dust....a couple of Iso. spray soaks and GENTLE, non pressure wipes cleaned 99% ; then a "usual" clean with the Isopropyl/ paper towel got them 110% No problems since then.
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