May not be necessary for a few months and yet…
I’ve been using this and it’s good stuff http://www.nikwax.com/en-gb/products/productdetail.php?productid=248
What I’ve found is that it’s worth coating the brow/crown of the crash helmet too. This means that (in my experience at least) the rain moves off the helmet and doesn’t get a chance to drip inside when the bike stops/slows.
I use Rainex.
I guess it comes down to price but my bottle bought over 10 years ago is still half full.
Cost about half a Pavroti.
The Nikwax bumph doesn’t say use on the inside of the visor - but anyone tried it to stop fogging.
Always interested in that being a spectacle wearer!