To paint over ANY surface, without sanding, use a product called 'ESP Easy Surface Prep', available at hardware and paint shops. Wipe on, wipe off and paint in 90 minutes. Works well over enamel paint , without having to sand. SAVES HOURS of work. It was recommended to me by a professional painter.
When choosing a ceiling medallion use the numbers here to help you decide what size ceiling medallion is right for your room. Measure the diameter and length of the room. Multiply the numbers to get the square footage of the room. Divide the number by 7. The final number is the number in inches that should be your ceiling medallion diameter in inches. For example, a room that is 12' by 12' would be 144 square feet. The ideal size for a ceiling medallion for this room would be about 20".