We had burned a strip of bacon, and it became, it seemed, permanently bonded to the pan surface. The Kitchn's instructions are for doing a really deep cleaning of cast iron and removing all the seasoning. We didn't want to have to re-season the pan, so, rather than making a paste of salt and water, we just tossed about a teaspoon of kosher salt into the pan and scrubbed.
A few minutes later, the pan was cleaner than it's been in a long while. Now we're cleaning the pan with salt every time: when the food comes out of the pan, we toss in a bit of salt, and when the pan cools down, we wipe it down with a sponge, then dry it on the stove.
We'd been holding on to Brillo pads for tough cast iron cleaning needs, but the salt works just as well, if not better. That's one more cleaning product out from under the sink!