My bathroom is under renovation. I put in a new floor, john and sink. Upon tackling the ceiling, my wife did a great job scraping, in parts, the large plaster swirle ceiling down to the "Green Board" used in bathrooms in the 60-70's. My question is what if anything do I do to prep the green board? And should I mix plaster and apply right over the green board. Or do I use joint compound. We want to replace the scraped parts/patchs with the exsisting plaster swirle