Save hand-washing house guests the horror of having to stare at a line of dirt and dead skin ringing the bathtub.
You will need:
clean, dry cloths
abrasive bathroom cleaner or baking soda
a scrub brush with plastic bristles
a spray bottle
a bottle of white vinegar
Tip: While you are at it, might as well scrub the rest of the tub - there's dirt there you can't see.