Our 1910 bungalow has some water damage over the fireplace wall near the ceiling. There is an area about 2 1/2 feet wide and up to 2 feet down from the ceiling that has shrunken about 1/4 to 3/8" away from the bottom of the crown molding. A crack borders each side of this area vertically. It looks like the old plaster has "caved in" a bit in that section. What needs to be done to the old plaster to make sure that a patch holds? Or do we need to just fill in with a couple of skim coats? We figured that the aluminum paint we used in 1973 to cover a water stain will need to be sanded off. The plaster is in good condition otherwise in that area.