If your looking at weathering the screen I'd sudjest the following
1. Look at the wiper arms and measure the lenght
2. Using this as a template cut out an arc to fit the screen on masking tape
3. Apply this to the screen with the apex at the mounting point of the wiper.
4. Spray or paint a VERY thin layer of matt varnish to the screen
This will give you the look of the wipers having cleaned the screen many times and dirt/dust building up outside this area. Its a subtle effect and I think more life like as you should still be able to see through the screen as you would in real life
Hope that helps
Pmitch