Klaus,

Sometime ago, I tooke the lenses out on my 1994 850 to clean them. I saw that the reflector was a little foggy with a very light dust like layer on it.
I got a very soft painter brush ( One of those used by picture painters, not wall painters) and carefully brushed all the surface. No damage at all. On some more stuborn spots, I actually used a piece of cotton for a more thorough cleaning, after brushing the spot. It looks like new. That was six months ago, and no side effects so far. The silver coating did no come off, but it can be scratched very easily. I think one might be tempted to use compressed air. No good as the air will have some oil in it or some humidity. This will really mess up the reflectot beyond hope.