Removing Holly Tree Sap from Glass Patio Doors

We just moved into a townhouse with a large holly tree in the back yard. Our patio doors have spots that look like water spots. We have had some success with Windex, Goo-Gone, and a razor blade. However, it is a very time consuming process. Since the holly tree branches cover the whole back yard, I can only assume that it is caused by sap or something from the tree. Might there be a specific chemical that can remove these spots?

By Richard