Colin Wilson coughed up some electrons that declared:

Hydrofluoric acid which is deadly and should be steered clear of.

I knew there was an acid that did it, but couldn't remember what it
was - it's probably also a biatch to get hold of...

It's very very nasty. Not at all like conc. nitric or sulphuric. Something
about where the fluoride ions go after they hit your skin - and how fast...

Apparently involves having appendages amputated if not flushed off quicky.
Not a chemist - just read the safety leaflet once.

google "hydrofluoric acid burns"

I'm totally mad, but I wouldn't go near the stuff.

Cheers

Tim