I've tried everything and now I just sponge the visor inside and out with water and a bit of dish-washing liquid on it then dry it off with a paper towel. Then when it rains like Wombat I just wipe my left gloved index finger across the visor for light rain. The index finger of the glove has a chamois insert the length of the finger.
For heavy rain I find it's better at 100km (on highways) than slowing down, the wind keeps the visor clear. Moving your head from side to side now and again also clears the visor of rain drops.
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Ride on !
