I have also had luck, with baking sheets, with coating it in *bacon grease* and letting it sit, warm, for a day or two. Scrape with a plastic scraper, then wash as normal. Has gotten a layer of burnt off stuff off all the way to the metal.
ETA when I say warm, I mean, I leave it on my gas stove, over the pilot light, so it's warm to the touch but not hot. You might want to try putting in plastic and putting it in a sunny spot if you have an electric range.