There is one VERY IMPORTANT detail to covering flats: you must use FULL STRENGTH Elmer’s white glue. Use a brush or small roller to apply it to the perimeter of the frame, then put the muslin on quickly before it starts to set. Smooth the muslin over the glue lightly. Finally, take a small pail of water and some muslin rags and work the muslin firmly into the glue with the wet rag. If you do this properly you'll get great results. Unfortunately many people like to water down the glue. If you do that then the muslin will shrink back from the edge when it's sized. As for sizing, we just used a very watery coat of white latex with a little pigment added. I would NOT use the other technique you describe of wrapping the flats. Our scenery goes in and out of a lot of trucks, and wrapped edges get torn and frayed. They also make lousy joints when they meet adjacent pieces.