When I buy second hand discs, they usually have some dust particles on the disc surface and at the bottom, which I usually can easily wipe off with a cotton swab. I use ethyl alcohol to clean the hard-to-remove spots, but sometimes I can't remove it. I even saw discs with a few small holes in the surface, strangely enough, the discs were still working and heard no interruption or something (maybe it has to do with the error correction included in MiniDisc). Well I hope the laser is strong enough to read through some dust. I think the best method is compressed air to clean MiniDiscs. Unfortunately I do not have an air compressor at home, but I think there are air spray cans available commercially.