We have a circa 1970s home with electric wires embedded in the ceiling, controlled with zone heating. A few areas had long cracks in the ceiling plaster. We had a fellow who claimed he knew how to work with these ceilings repair the cracks, and they looked great for a few months but the cracks are now back. The main ceiling is made of plaster (not drywall) and he mixed up something to repair the cracks. Is there some type of special hardener or heat-resistant plaster that would make a more durable patch? Taking down big sections is not possible because the wires are thin and cutting any would cause the whole system to malfunction. Any info would be appreciated.