Last weekend I was out on a photo shoot, everything went fine, until I got home and saw something on my circular polarizer. I attempted to clean it off but it just kept smearing around. At first I thought it might be sap, but it wasn't warm enough and I didn't even get that close to the trees for it to be sap (besides, my lens cap is on when I'm not shooting).. anyways, I just can't get it off. I don't want to scratch it, so I haven't tried very many different methods of cleaning, but does anyone have any recommendations? Or can you take filters into camera shops to have them cleaned?