Clean your bathtub inside and out with a strong cleaning agent and scrub brush. Scrub heavily if needed to remove dirt; you needn't worry about damaging the tub's finish.
Let the tub dry completely.
Remove all caulk in and around the tub with a putty knife.
Wipe the entire tub down with a clean cloth and acetone to remove soap scum.
Sand the bathtub with 80-grit sandpaper to create an even, clean surface.
Scrub all rust away with a stiff wire brush.
Repair all nicks, grooves and damaged areas using putty or an acrylic repair kit.
Cover fixtures with painter's tape to keep them free of paint.
Spray a coat of acrylic spray paint primer on the bathtub, starting from the drain area and moving toward the back of the tub, and moving from the inside out. Let the paint dry thoroughly before proceeding.
Spray acrylic paint in the color of your choice, using even strokes and keeping the can about 6 inches from your body.