Vegetable oil. Take it outside, set it in a large bucket or wheel barrow, then drizzle some oil over it. Using a stiff brissled brush, scrub. Pour a bit more oil. Scrub. Eventually all the rust will be off. Do NOT dump the oil into your yard/garden. It will kill everything. (If you have some bad weeds you can kill them with this.)
Place the used oil in something like an old empty paint can or jar with a tight lid, then discard. (To be properly environmentally safe, take it to a "Hazardous Waste Facility" that accommodates "Household Hazard Waste" Here in Los Angeles, only a few take my insulin syringes.
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