The DU Lounge

straight white distilled vinegar in a plastic bag. Soak for an hour, see what comes off. Scrub with steel wool. Rinse, dry, repeat.

Alternative to vinegar is (you may or may not believe this, but it works) soak it in Coca-Cola. Be sure to rinse it well after you're done, b/c the sugar will gum it up.

If that doesn't work, try an electrolysis bath. More expensive, moe complicated, but not hard, and guaranteed to clean it. Plenty of YouTube videos on how-to. I use it for cast iron, and it works great!