You can use WGU on any part of your car except ya know, the tires. With glass, the trick is to work it in thoroughly so you're filling all the small pores to seal the glass fully and then make sure you buff off thoroughly. I would jump inside the car and take a look too just in case you miss a small amount of streaking or smearing. That's why it's more expensive, its' a lot more versatile. Iron X is the safest decontamination method essentially, especially for a coated car, the idea is that yes you CAN clay if you would really like to but it's a risk issue. For most coating installations, the paint has been absolutely polished to the best that the person can so do you really want to take a risk in marring after all that? I mean ultimately of course it's upto you but it's not suggested after all that work.