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.
I’ve never had it go through the bag, but I always use 2 bag layers, taped shut, at least, as a precaution. Yes, the lye CAN eat holes in the bag, since plastic is made from an organic material. I personally have not seen it happen.