My D3 sensor got absolutely caked in dust to the point I had to wet clean it to get rid of them, even after using a rocket blower. I got a kit from hilton photographic with 6 swabs and it fully cleaned it after 4 of them so its now back to being clear. Its a lot easier to do than I thought and quite fool proof really. Use the right amount of solution and don't go over an area you just cleaned. One side of the swab is the wet clean side, whilst the other wipes residue from the sensor. When you get use to the "stroke" you need to use to get the swab to glide across the sensor, its VERY straight forward.