I remember being told that dry ice works for undercoat removal so I searched on that to be sure it wasn’t an old hallucination! Found this:
https://m.youtube.com/watch?v=wcuLPO36Rmo
There were other videos and info as well, this was the only one I watched. This one shows whacking the metal with a hammer and while you wouldn’t want to dent your door with a BFH it might be worth trying with a softer rubber mallet?