My sister happened across an old cast iron skillet at a garage sale a few weeks ago. This is an old Griswold pan, not one that you can pick up from Lodge on Amazon.
And it was in really bad shape. Lots of crusted on gunk, no "seasoning" at all, not safe to use as a cooking instrument.
She called and asked for some input on how to get it back to its former glory.
First thing you need to do before you ever think about reseasoning the skillet is to start with a clean slate. Given that it is unworkable at the moment, throw out any ideas you have about how to treat cast iron. We've all been told "Never use soap" or "Only wipe with a soft cloth".
Start with the hottest water you can stand, some heavy duty soap, and a steel scrubber (like a Brillo pad). Scrub, rinse, scrub, rinse, scrub, rinse. Get as much material off the iron as you can. You may want to let the skillet soak in a sink of hot soapy water for a few hours, then scrub more.
Once you've gotten as much off by hand as you can, dry well and head for the back yard. Spray all of the surfaces of the skillet with heavy duty oven cleaner (wear protective gloves!) and make sure you get a good heavy coat of the oven cleaner on the skillet. Put it into a plastic trash bag, then put it into a SECOND trash bag -- the 13 gallon size kitchen trash bags work fine -- and tie it closed. Put the bagged skillet in a spot that will get as much sun as possible, and leave it alone.
The next day -- yes, leave it sit for 24 hours -- open the bags, remove the skillet, and wash it again with hot soapy water. Inspect for any residual material on the cooking surface, but there shouldn't be any.
At this point, you should have bare cast iron and nothing else. Every bit of material should be gone.
Get your oven going, about 375 Fahrenheit. You can also use your outdoor grill, if you prefer. Get yourself a pound of unsalted lard melted down (don't use cooking oil like vegetable, canola, EVOO, etc.) and wipe the cooking surface with the lard. Go ahead, use a good amount, it won't hurt. Put the skillet in the oven or on the grill, let it get good and hot. Wipe the lard around some more, and let it sit in/on the heat for about an hour.
Repeat a couple times. I find that it generally takes about 3 or four applications to get a good season on the skillet.
After 3 hours or so of heat and lard, turn the heat off, but leave the skillet where it is. Let it cool down slowly. Once cool, give it a quick wipe with a clean, dry paper towel to remove any residual lard, and you're good to start cooking with your newly seasoned cast iron skillet.