Dirt and soap scum will prevent primer and paint from adhering to a bathtub. Scrub the tub with a heavy-duty cleanser containing trisodium phosphate, prior to application.
Cover areas you don't want painted, within and adjacent to the tub, with masking paper and professional painter's tape.
Bathtubs will not accept a new finish unless they are abraded with sandpaper first. Scour the tub with 120-grit sandpaper. Once the tub feels slightly coarse, it is ready for a base coat of primer.
Apply an acrylic latex primer to the abraded tub before painting. Although a roller and paintbrush can be used, spray painting will generate a smoother, professional-looking finish.
Acrylic latex and oil-based paints are not appropriate for bathtubs and will eventually chip and peel. Use an appliance epoxy paint on the bathtub. This coating provides an attractive sheen and a high degree of durability.