Best way to get "caked-on" residue from VERY old, rot-iron pans?!?

Put a small amount of water (1/2" is plenty) and boil until the crud loosens. Then scrub it out. If it's really stubborn, you may need to do this more than once.

Once it's clean, put the pan back on the stove to dry completely, then while it's still hot wipe the inside with a light coating of vegetable oil. Cool and put away.

If you get the same sticking problem next time, your pan probably needs a thorough cleaning and reseasoning.

I put mine in a fire pit for a couple of hours, then wash, coat and bake for an hour or so.

What is a rot-iron pan?

If you have a self-cleaning oven put it in the next you do it. When it's done wash it good, dry it on your stove top, wipe it down with vegetable oil and put it in your oven for an hour or so at 250 degrees.