Rain X is great for Glass. As on your car or truck Windshield, Side windows and Mirrrors.
I would not apply it to a Face Shield. Turtle Wax, McQuires or Similar Canuba wax products will allow you to just briefly turn your head left and right to see clearly on your next encounter with riding in the rain.
I use 3M car wax as it seems to be readily available, and has not yellowed or hazzed any of my faceshields as of yet. : )