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Basically, you scour off any rust, coat with a good vegetable oil - put in the oven at 250 degrees for a few hours, then let it cool. Repeat. Then only use for frying for the first few times as this is part of the seasoning process.
That's good Azz , one way to clean a very dirty seasoned pan without using detergents is to rub it lightly with salt and a piece of kitchen roll before washing in warm water, then dry thoroughly.