I live in a 1900 stone-built house. In the living room we have a bay window with a plaster moulding above the window, where we want to put a curtain rail. In some places, there seems to be gap behind the plaster and the stone wall is about 2-3 inches further behind the plaster. The window is quite high and curtains are quite heavy so any fixing on the plaster is likely to degrade and fail with repeated opening and closing of the curtains.
Is there any kind of exanding foam that could be injected behind the plaster that would bear enough of a load to stop the curtain rail from falling down?