I have always used Barr's too, it don't clog radiators and heater cores but I just heard a friend from a dealership say they have some kind of powdered ginger that works really well. On something as small and easy to fix as a thermostat housing, I would take it off and fix it right. Check it for flat with a straight edge, lap it with a sheet of good sand paper or emery and a real flat surface, like a pane of glass.