Clean as said, lightly rub down and yes put a undercoat on it is a lot more work but it is the undercoat that helps to give you that better finish, and well worth the effort,as with all decorating it the preporation that is important,TIP dont hold your brush horizontal too the surface hold brush at a diagonal position this will help to aviod brush marks, take your time and dont rush just to get finished, this is a common mistake, a rushed job is aways a bad job!! good luck