Clay is brilliant on windows, Give them a quick clean with glass cleaner then clay and then a final clean again. Clayed mine yesterday and even without rain-x they are repelling water quite well.


Edit: Chris ain't wrong about glass polishing, I had a go at my headlights using cerium oxide and a drill bit and after hours of work I only got about 20% of the blemishes out - I don't fancy doing a large glass panel at any stage in my life