Our most used cast iron skillet had a tenacious crust of burned-on grease and carbon that had collected over the years, and because the surface of the bottom of the pan was no longer smooth, many foods would stick. Initially I thought the unevenness was the result of corrosion, but no. I took a sturdy stainless steel spatula to it and with quite a bit of elbow grease over a few days managed to scrape off all the crust and get down to the smooth surface underneath. Now anytime anything sticks and the surface isn't perfectly smooth, I use the spatula with a little water and Dawn dishwashing liquid and quickly scrape until it's smooth again (it's easy to feel with the spatula), followed by my normal routine anytime we use the pan: drying the pan with a dishcloth and then by heating it on the stove, wiping the surface with a few drops of olive oil, and leaving the pan on low heat for a few minutes before hanging it back up on the pot rack. Works great!