I have a polarising filter made by B+W. I have used it for darkening skies and it worked well even though it is single coated. In fact, single coated filters seem fine generally as long as i am not shooting straight into the sun. My Canon FD 50mm f1.8 is only single coated anyway, I believe. On a tripod I often shade the lens with my hand (my rubber shade fell to pieces some time ago. I guess i was just asking the question out of interest as SMC lenses have a good reputation.

Dave