What is the best way to clean an old, white, cast iron farm sink?

My husband and I bought a Queen Anne home built in 1903 and renovated it for our office. It has a tiny kitchen, added much later I'm sure. It has a cast iron sink with drainage grooves at one end and the sink on the other end. It is badly stained but no one sees it except us, so I don't want to go to the expense of having it resurfaced if I don't have to.

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