I've heard that Hoya filters are difficult to clean.... I just received my new (sealed) ND400X filter, and the glass is rather smudgy looking. Instructions say to clean only with a soft cotton cloth, no cleaning solutions. I've tried cleaning it with a new microfiber cloth, but all it does is move the greasy smudginess around. Does anyone have any tips on how to clean the filter without leaving it full of streaks and smudges? I breathe on the lenses in my glasses and then clean with a microfiber cloth, but that method doesn't seem to work with this Hoya filter. Advice please?