I've got an Ox to grind - worked on the tailgate today
Turns out most of the crusty stuff is paint curling up. The top layer is a skim coat and white, below that is a dark brown, followed by a dark green and then finally a light blue. I'm using an angle grinder with a flapper disk with light pressure. When I hit the green I know it's time to ease up because the blue is underneath.
Not sure why I'm keeping the blue, I guess it's kinda cool the original paint has hung on so long and with all the others it's managed to keep the steel in surprisingly good shape.