We have a small bathroom at our beach condo (10 x 5). There is no window. The unit was built in 1990. The ceiling was painted some sort of antique white to match the walls — that we now think is oil based. We painted over it with Sherwin William Eminence acrylic ceiling paint. No primer. The ceiling produced a streaked, discolored finish. We bought some pigmented shellac primer sealer; and, when we bought it, the paint clerk told us we had to sand the entire surface before we use it. Do you agree we need to sand the entire surface? Can we just put on the shellac primer without sanding and then put the product back on it? If we must sand the entire surface, what is the best and easiest way? We just refinished this bathroom.