I am quite experienced in cleaning FF sensors. However, trying to clean a Hoya CPL filter proved to be too bad for my nerves... It seems that instead of removing dirt, I was just moving it around. I used a microfiber cloth and lens cleaning fluid by Kodak. Then I tried a pair of the wet/dry tissues that do a fabulous job on my monitor. After 15 minutes or so, I was still not satisfied at all with the result. Is there a reason some filters are so difficult to clean? For example, I find B+W filters much easier to clean. I also have no issues with my Singh-Ray polarizer. Any advice on the subject would be very welcome.