http://www.bellracing.com/faq/fq.htm
What is the proper way to clean a face shield?
Please do not use paper towels or thinners of any nature. A mild soapy detergent (preferably sprayed on) and a soft rag will be enough to clean the shield. On our chrome shields the rag must be extremely soft and no harsh rubbing action should be used (pat it dry). If a visor strip is applied to a chrome shield it cannot be removed without damaging the coating. Use extreme care and absolutely no solvents.
You might call a motorcycle shop for some alternatives.