Seen how a clay bar can remove various substances from paintwork to produce a glass like finish. Hence my question, could you use a clay bar on a windscreen to remove all those contaminants that stick to the glass and thus make the wipers work even better ?

I've done all my windows.

Was the only thing that would remove the tree sap that was on it when I got it. White spirit didn't even move it......

Rain runs off pretty well after as well.

 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

Thanks for that. Think I'll give it a try as glass is bound to pick up tree sap and tar spots swell as the paint work

Bar keepers friend is excellent for cleaning glass  Present :2008 TDV8 HSE
Gone Audi A5 2.7TDI
Gone Discovery 3 HSE
Gone Mercedes CLK
Gone Range Rover 2.5 DSE

I used 0000 grade steel wool, it brought the glass up lovely and smooth. Then I used Rainx and my screen is amazing. Then I did the same on the inside. Now when I get in on a cold day there is no misting.