My girlfriend bought a cafe and they had a cast iron waffle iron that had, in some places, 5mm of grime built up over 2 years, never once been cleaned. Even the hinges were full of old crusted on batter and grease.
I spent hours with a grill scraper scraping every individual pin.
I have gotten to or past the seasoning in many places except in between all of the pins is hell to try to scrape the old baked on grease away. I have tried sodium carbonate soaks, a wire brush and a wire brush on a drill, a few different scraping tools, and I have only gotten about half out of those grooves. Miles better than before, but still not perfect enough to season and put back in the cafe.
You can also stick the cast iron in an oven and use the ovens self cleaning mode.
I would not do this, as heating the cast iron in this way while it’s dry could warp or damage it.