If you check most directions for products like rainex and cleaners like Windex with ammonia base, I think you will find that is mentions about use not using it on plastic windscreens.
Steve, how long does the FogX preparation last? I know without it, going through California's central coast in the hills and valleys, I had to deal with on again off again fogging that drove me crazy last fall. My acrylic faceshield was a small area, but I was sure glad I wore soft deerskin gloves (with no seams) so I could wipe it off and dry them on the rear cylinder cover.