If the windows are not 100% clean u might get some smearing and hazing. Use VERy little Z2 on the windows. Wash with Z7 wash to level it out and you will be ok if hazing does occur. DO NOT use Z1 or ZFX on the windshield, works best with pure Z2.
I use rainx on my front windshield now but Z2 on the other windows and it works great. It greatly adds to the detailed look of the car with nice clean reflective windows. It also makes cleaning a breeze. I never use window cleaners, they come out perfect and streak free after a Z7 wash, and I z6 them to aid in less dirt pickup and slickness. Zaino glass polish is a great prep to use before Z2ing them.......
Hope this helps......