Depends on the material. Silver3d you don't really want to if you can avoid it. Most of the materials can be cleaned with a mild soap and water solution. Dry sabbing the problem area rather than wiping if possible. If it still won't come clean, you might call tech support and see what they suggest.
Stewart Silver 3-D screen shouldn't be advised for home use, IMHO. It's a special-purpose screen (for Polaroid process 3-D films, of course) and it can not be cleaned without damage to the surface. It's that simple, folks. Even commercial theatres don't attempt cleaning it. Splotches will result.
That said, if you want to see a great series of original 35mm Polaroid process (as opposed to alaglyph red/blue) 3-D films (projected on a Stewart Silver Screen, natch) there's a 3-D Festival going on at the Egyptian Theatre in Hollywood. It's great!
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