Eldest held a sledge for me whilst I used percussive persuasion with another sledge. After 5 minutes with the mapp torch. Without heat it didn't budge, but the heat did the trick. I'll put some spray zinc on for now and paint it all up when I do the inner door posts.
Corners are tricky, back to sandpaper and hard work. The rest I'm mostly able to do with powered tools, including the katsu rotary tool.
I'm very tempted to try the stain removal gel again, there's a lot of smaller grey stains that annoy me. If I rinse thoroughly, and leave to dry it should be okay.