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 (from website).
How to paint bathroom tiles:Use the Rust-Oleum Specialty Tub Tile Refreshing Kit acts and looks like porcelain and ceramic. This specialty epoxy paint product is recommended for tiles that have a high glaze finish, and where oil-based paints will not properly bind to the surface of the tile.