I thought RainX was not recommended for use on plastic? No?
The RainX bottle says don't use on plastic unless the manufacturer first approves it.
I decided to experiment on a small area out of the line of sight.
There was absolutely no ill effect.
Since then, I've been using it for years on Shoei, Bell and HJC face shields, and never have had a problem with any of them
Not only that, because of the easy hand only cleaning, and aside from rebuffing with a clean microfibre cloth after every washing never touching the shield, my shields seem to last forever.
If fact, I have never bought a replacement shield for any of our helmets since 2005.
We have Clears and Smokes for each.
True, I'm not a mega miler, but being fiendish about faceshield protocols makes them last longer while you see better through them, be it wet, dry, night or day.