Now that summer is upon us, it is also the season for baked on fly guts to adorn on finely polished windscreens, now Im not sure if anyone has used this, but I discovered that using some fine wire wool with some Windowlene (other glass cleaning products are available) removes the guts successfully with very little elbow grease involved, and works well at removing tar and other deposits. With glass being harder than metal there is no risk of the windscreen getting scored or scratched during this process. Just bear in mind, DON'T use it on your paint work!
Happy Cleaning!