Here are the directions from Country Living:
Prime: Thoroughly clean the tub's exterior, then apply a quality latex primer with a foam roller and let dry to create a smooth surface.
Apply: Paint at least two coats of exterior-grade latex paint in a satin or semigloss finish onto the exterior using a foam roller.
Set: Some paints may take 90 days to completely set. To prevent marring the surface, avoid scrubbing or scratching the exterior.