I was told to use Mr Sheen (or any beeswax furniture polish) when cleaning my lid.
Worked ok for me, it does help bead water on the visor.
I have also used Nikwax Visor Proof as well. Its good, but if you wipe your visor you remove the layer off the visor, and its water based, so it will wash off anyway.
I have found that if it is raining and you have a product on your visor, generally speaking, they all work, but as soon as you get mist spray off a lorry, you are cream krackered as it just sits on the visor forcing you to wipe it with something.