painting filled crack in bath
I would use acrylic paint, marine paint or white appliance enamel "touch-up" repair liquid.
I've always just used the "bright white" enamel touch-up repair liquid for my white tub/shower units. It doesn't exactly match (whiter), but it drys hard as a rock. It can be found at many appliance stores.
Clean the scratch, crack or ding with acetone (nail polish remover).
Dry it with a hair dryer. Apply the touch-up liquid and smooth up the edges with a Q-tip dipped in nail polish remover.
Let dry overnight. It shrinks a bit, so you may have to apply two coats. Works for me.