Getting the windscreen back to being a lovely, flat surface is actually not that big of an issue when you use the right gear. Our Naked Glass is a brilliant glass cleaner, but it does not have the oomph to get in there and remove a lot of these other articles bonded on the windscreen. However, there are two products, seemingly hidden in our car care arsenal, which work brilliantly for cleaning up glass and windscreens, both you would never expect to use for this task; Namely our Fine Clay Bar or Medium Clay Towel and then the Metal Polish. The clay bar or towels' job is to pull hard particles off from a hard surface (which glass certainly is), so a lot of the specks and marks bonded onto the screen like tar, bug marks and overspray can be removed quick and easy. I presonally prefer using the towel as it works a lot faster than a clay bar.
For some of the harder things to remove off the glass, our Metal Polish is just fantastic. It uses a jewellers rouge which is brilliant for the deep cleaning of glass, removing a lot of marks the Clay Bar or Clay Towel cannot touch, even removing light water marks, which can be quite annoying to clean off normally. If used with a polishing machine, it can even remove light wiper marks and scratches from the screen as well.
Using these as a system is a good idea for at least once a year on a daily driver, and as required on the more babied cars you have. Make a note that the Metal Polish, Clay Bar and Clay Towel are not for perspex, plastic or tinted items though, as these surfaces are soft and can be marked/damaged quite easily.