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.
How to: Paint cabinets (secrets from a professional). All the tips and tricks you will ever need to know straight from a faux painter. Two-toned cabinets, yellow on the top, blue on the bottom, with antiqued glaze finish. Theraggedwren.blogspot.com