Depends what brand.
Filters vary from Sirius filters (get them wet and the coating flakes off!) to Astronomik and Baader that are rock hard. I breathe on them with a good "Hhhhhhh!" and rub them with a cleaning cloth and they come up perfectly. You can also do this with genuine Lumicon, but not sure how good they are now Parks manufacture them.
Regards,
Rob
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