If you're using the plastic, it can go in as is. Allow it to cure as stipulated in the directions. When the plaster is ready, remove the cup, then sand and smooth the plaster as desired. Do not do this before it has hardened sufficiently. A commercial emery board works well for the smoothing.
Paint takes better if a coating of gesso is applied first.
Models Featured On-Air Painted red and decorated with bands of gold stickers and geometric shapes. Short bookends were painted and made to appear like tables, with three short lengths of dowels down the sides.