I have found when dealing with glass it is not the glue so much as the surface prep. You need to apply hydrochloric acid to the glass (tape off carefully). This will fog the glass and allow the epoxy or glass glues to bond.

There is a newer way to remove sap and other contaminants from paint. Griot's garage(and other's) sells two different surface prep mitts that you use with speed shine. It serves the same function as a clay bar but way, way easier and better. The mitts are expensive but worth every penny. The mitt co...