my 'nuclear option' for cast iron skillets is to put them in a basin of lye solution, and let the pan sit for a week or a month (depending on how fresh and how concentrated the lye solution is.) We keep a plastic basin out back for this. lye is bad stuff, so if you have kids or pets, do please be careful. The lye, however, will take off anything organic, and bring the skillet back to bare metal (well, some scrubbing may be necessary. use a plastic scrubby). in this condition, the skillet will start to rust instantly. I work as follows: after the four weeks, take the skillet out of the bath, scrub it and hose it off, then immediately pour white vinegar on it: the acidity prevents rusting. Then put it in a 450 F preheated oven for half an hour. Then start the reseasoning process.
As I said, this is the method of last resort.
Here is a photo of a #3 cast iron pan, rolled back to 'year zero' after a few months in a lye solution, next to a seasoned #2 pan. The iron actually looks like metal. In this condition, you can check the surface for pitting, cracks, etc. The vinegar is on hand to prevent rust.