So I just recieved my 70-200 f4 and had my first test run with it this weekend. Went out to the dirt track with my friends to shoot them riding. Problem is that the track was super dry, so there was no shortage of dust clouds. ATV's made the problem even worse, where I gave up shooting mid day. I was using a filter to keep dirt and grime away from my glass, and made sure never to detatch the lens from the body. But now im paranoid some grime got into the manual focusing ring, it seems like it sounds different. I know air in a can, and a blow gun from a air compressor are out of the question. Am i SOL?