Most of our sessions painted on contact paper applied to the windows beforehand, but some painted right on the windows. When wet, the paint wiped up easily from surfaces but we have yet to wash the windows (I'll post an update when we do letting you all know how clean up goes).
We used Colorations brand finger paint and did test the process out beforehand. We used craft sticks to dish paint into leftover microwave meal containers. Do be sure to put away your supplies or prepare to have families self-serving more paint and using the craft sticks instead of fingers. Some of us laid down paper on the floor in front of the painting to catch messes and some also offered aprons for the kids to wear.
As some of the more heavily painted spots dried, the paint flaked, but this is a case of process over product anyway. It doesn't look as nice from the outside as some of our window activities, but again, process. I think the preschool and family storytime staff weren't a huge fan because of the mess, but the baby and toddler storytime staff had a grand old time. A difference in expectations, perhaps? The kids loved it almost universally. I had the mom of a toddler tell me her daughter usually hates messy play but couldn't get enough of this activity. Success!