The best plastics polish (with light hydroscopic properties) I've found is PLEXUS. I use it on all screens and visors.
I was put onto it by a guy who restored/maintained aircraft. It does NOT leave a film on the surface like Rain-X. ime DUST sticks to RainX.
I rode 450kms at 145kph non stop in a huge weather front that covered Spain at end April, and ne'er a droplet in my vision (OK except one that found it's way through the vents onto the inside) - Plexus protected.
The other option - but is trickier to find the correct mix is to find a PLASTICS specific Nano-coating.
Most are for GLASS and they are not the same.
One example
http://percenta-nanoproducts.com/product/nano-coating-for-plastic/
This basically puts a molecular thin HARD skin of Titanium and Silicon oxides on the surface.
I have used this type of solution in the past - my experience was positive - but you will need to retreat more than you'd expect. I think vibration is an enemy of this solution tbh. The TiO2 is basically a catalyst so resists biological infestations once the sun hits it. i.e kills any bug residue.
Nano coatings are very good if you get lots of light rain which means you get dirt on the screen/visor. It basically reinforces anti-scratch.
Polishes in general fill any existing scratches. Nano's in theory do as well but imo not AS well as basic polish.