If you cook with it a long time and it gets a thick crusty coat on the outside from only scrubbing or wiping the inside you can build a charcoal fire and put in it and fill it with charcoal or get it red hot with a propane torch. Look like new after, grey all over. Re season and continue. Makes no difference. ear
I use a safer method and works well. I fill up a large plastic container with hot water and add one tablespoon of Arm and Hammer Laundry Soda per gallon. I take two 1/8" mild steel plates (should be at least 12 inch square or larger) and position them on both sides of the container, connect a copper wire between the two plates with mild steel fasteners (strip some insulation from the wire in the middle), then I take some bare copper wire and connect it to the fry pan (clean an ares on the handle for a good connection), wrap the copper wire around a 2x2 piece of wood (non-conductive) and lower the pan into the solution between the steel plates making sure the pan doesn't touch the steel plates, use the wood 2x2 to suspend the pan in the solution. I then take my MANUAL (non-automatic, non-computer) battery charger and connect the red clip onto the steel plates connecting wire and the black clip onto the copper wire twisted around the pans handle. Plug in the charger and in a few minutes you will see bubbles forming, in about 4 hours UNPLUG the charger (water and electricity don't mix) and check the pan, you may need to scrub it with a brillo pad and reconnect and let the pan "cook" a while longer. This method will also remove rust. Use the method for CAST IRON ONLY, NO Stainless Steel, chrome or nickle because of chemical reaction. Should be outside, produces hydrogen gas. You could search YouTube for "electrolysis restoring cast iron". The solution is non-toxic and can be disposed in your septic or simply poured out.
I am surprised at the number dudes who are worried about germs.
My grandparents were farmers and both grandmothers houses were spic n span but the in the food preparation, concern about germs and such was never ever expressed.
Wood cutting boards were just washed in hot water, although back then most hot water heaters had extremely hot water.
My dad especially pushed the limit on meat being on the edge of being edible.
I ate some chicken a week or so ago that was not cooked well enough and made me wonder if I should stick my finger down my throat but it finally moved on down the poop shoot after a bit of being uneasy and I am just fine.