Remove the grates from the stove, grill or fireplace. The grates should be completely cool when you remove them.
Scrub the grates thoroughly with a wire brush to remove as much of the rust, soot and food debris as possible.
Rub the grates with a steel wool pad then wipe them down with a moist microfiber cloth. Repeat until the surface is smooth.
Make a paste of equal parts coarse salt and vegetable oil. Heat the paste in the microwave for 30 seconds.
Scrub the grates with the warm salt and oil mixture applied with a clean, dry sponge.
Wash the grates in hot soapy water then rinse and dry them.
Coat the cast iron with a layer of plain vegetable oil. Replace the grates. For a stove or gas grill, heat the oiled grates for several minutes, before use, to create a seasoned surface that resists sticking.