Ben,
do not use dulling spray on your windshield. Use a polarizer to dial in or out what you want to see. also, if you want some ambiance reflection, (trees, buildings, etc) drive down a road with these things. you can dial in some of the ambiance while still mainting clarity on your subject.
the only time you want to use dulling spray is if you need to take down a small flare that is shining into your lens making the image milky or completly blown out. For this we try to use black paper tape. I personally like some flares. Its real.
lol
Todd McMullen
Flip Flop Films
Austin
http://www.toddmcmullen.com