"Unless you intend on using the iron, do not clean it. It actually lowers the value to remove the natural patina of age."
Couldn't say it better myself. Anything you do will actually diminish the value. If you don't care about the value everything I've read here does work. With sandpaper start with low grit and go up till you are using the metal grade stuff that is black. But more than likely you pry have a kinda cast iron one in the first place and that was from a mold and the iron isn't and never was smooth anywhere so it'll be very noticeable that you fixed it. You could also give it a nitric acid bath. Always a fast way to remove rust but it takes away the entire level that is on the surface, rusty or not. But I don't think I'd recommend that to someone who doesn't already work with metal.
But I've been "antiquing" since I can remember. My husbands grandmother even owned a huge antique mall. It's still my family's hobby even. Anyways, if you don't care about the value then mess with it but to be honest I'd just leave it alone. It's called a rustic antique. Rustic's are actually very collectible but .... only when not altered.