How To Clean and Season Old, Rusty Cast Iron Skillets...using salt and a potatoe. See also commenter about putting a rusty skillet in the fire.
My dad has used the same cast iron skillet for as long as I can remember. Once, he forgot and left it heating on the stove. It turned orange (yes, THAT'S hot!) and cracked in half. My dad's devotion was such that he took the two halves out to the garage and welded it back together. That was at least...
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Easy way to clean rusty cast iron grill grates.
Technique: Seasoning Cast Iron
How To Clean and Season Old, Rusty Cast Iron Skillets