Opticlean
In 2001 I had a minor disaster involving the dropping of a lens into the sands of Kalahari, with its rear element exposed and facing down! Gentle blowing removed the 'gravel', but I realised that any polishing would grind the lens surface horrendously with all the microscopic dust that was left. Having tried Opticlean, I am simply amazed! Needless to say, it removed all the remaining dust. But I also tried it on a newly polished (using lens cleaning fluid and tissue) filter which I assumed was perfectly clean. Well, it wasn't. Not by Opticlean standards, anyway.
British Opticlean is a liquid polymer that is applied directly to the surface in need of cleaning. When the polymer has cured, it is lifted off as a thin plastic film along with every speck of dust, thumb print and other dirt. As opposed to other lens cleaners, you do not scratch the surface. In addition, you do not get the residual smear, which is left by other lens cleaning fluids, that greatly affects the antireflection coating. The polymer is completely harmless to glass and antireflection coating, but should not be used on plastic objects. It may even be used as a protective coating during storage and transportation. One 6 ml bottle of Opticlean cleans approximately 20 standard 35 mm lenses.
A bottle of Opticlean costs SEK 245 including VAT and postage within Sweden. (For C.O.D. orders, the Post charges an additional SEK 40.)