Potato and soap, salt and lemon, or baking soda or vinegar.
For potato, cut in half, put soap on cut potato end, and scrub like it’s a sponge.
For salt and lemon, salt the rusted surface, cut lemon in half, then scrub with a cut lemon. You can also use a lime.
For baking soda, wet the metal, shake dry, put baking soda on the rusted parts. Let sit for an hour, then scrub it with steel wool or metal brush.
For the vinegar, use white vinegar, soak the metal in it overnight, then scrub with steel wool or metal brush. If it’s too big, you can wrap it in towels soaked in vinegar.