It happens even to the most diligent cooks—you turn away from your eggs for just a moment and they're burned to the bottom of your cast iron pan. Or a few errant water spots have left your pan polka-dotted with a layer of rust. Don't panic! While a cast iron pan can crack irreparably if you place a searing hot pan in cold water to clean it, most other issues have an easy fix.
If burned bits of food have adhered to the pan:
For a gentle but effective scrub, take a tip from the spa and use salt! Generously sprinkle kosher salt or coarse sea salt into the pan and scrub with a clean, damp washcloth or paper towel. For extra stubborn spots, use a plastic pan scraper to work the burned food off the pan. Stay away from steel wool, scouring pads, or detergent scrubs, which can remove the seasoning.
If your pan has rusted or if acidic food like tomato sauce or vinegar has removed the seasoning:
Use a gentle detergent scrub such as Bon Ami or Bar Keepers Friend to clean off the rust or the acid-damaged spots. Rinse well and dry thoroughly.
Rub the unseasoned, scrubbed parts of the pan with vegetable oil, canola oil, or melted vegetable shortening. Depending on the number of unseasoned spots on the pan and their location, you can reseason the pan on the stovetop or in the oven.
For small spots on the interior of the pan, place over medium heat on a stove burner and heat for 10-15 minutes, until the oil has baked on. Turn the burner off and leave the pan on the burner until cooled completely.
For larger spots or if the spots are on the underside or exterior of the pan, reseason in a 350-degree oven as instructed here, placing the pan upside-down on the top rack of the oven and baking for 30-45 minutes.
If you want to start from scratch with a vintage cast iron pan:
This is the one instance where you'll want to go to town with steel wool. Scrub the pan inside and out to remove any remaining seasoning (and anything else that's been baked onto the pan over the years) until you see the natural unfinished gray-blue iron.
Wash the scrubbed pan with gentle dish soap and water, then dry completely. Follow these seasoning instructions, repeating 2-3 times until the pan has blackened to a familiar cast iron sheen.