Plexus plastic cleaner can be bought at the sportbike shop. I used to use protect all or pledge on the face shields of my helmet to keep it clean. Full face only in the rain for me. I don't like razor blades hitting my face all day long.

 

H-D part number 99841-01 Windshield Water Repellent Treatment, about $7.00 for 10 treatment sheets. I have not tried it, but I would use it before RainX.