I have a home built in the ’60’s. I think the walls are sheetrock(?) (what looks like cement on paper backing with thin coat of plaster over surface). The Living room and Dining room have

large wall to ceiling cove made of plaster, I’m guessing. The upstairs office has this same treatment, but only on one wall. The opposite wall/ceiling intersection is at a right angle. I would like to match the cove. I have seen styrofoam cove forms advertised that are then skim coated with joint compound I imagine, but I was concerned about telltale hairline cracks where the cove ends are feathered into the wall and ceiling. I’ve also seen the cove form built up with scrap wood covered in a metal screen, then shaped with progressive coats of compound using a piece of sheet metal cut to match existing cove’s radius. Any suggestions? Will joint compound work or would I be better off using a plaster mix or adding plaster of paris to the joint compound (I had a plasterer do this in a previous house for a quick drying, durable patch job) Thanks! WB