I'm sending my AF100 in to Panasonic (Canada) to clean dust from the nd filters...wondering if there is anything else worth tending to while "the hood is open".
How bad is it? v(isible to eye, visible on shot? greasy dust)
Blow gently with a can of AIR then gently with a photobrush, more gentle air then check.
I believe the sensor and the rotating NDs are behind a "clear filter" ...so you only have to clean that. (Some one should correct me if I am wrong please)