Objective: To clean rust stains off of the bath tub.

Which of the following solutions is more sound in terms of naive physics reasoning?

Options are:
(1). Use half a grapefruit and pepper. Sprinkle about a 1/4 cup of kosher pepper on a halved grapefruit.
(2). Use half a grapefruit and salt. Sprinkle about a 1/4 cup of kosher salt on a halved grapefruit.
(2).