I know this is an old thread but just to point out that Hydrofluoric Acid should not be used by anyone other than a competent operator.
My opinion - to suggest, on a public forum that someone asking the question what to use and it is suggested to use HF is completely wrong.
This stuff penetrates the skin and then 'eats' the calcium in bones. You may have no obvious skin break and no obvious external issues.
It doesn't stop when it hits bone it carries on munching away till the limb affected is useless.
As 'experienced operators' giving 'advice' to relative newbies we have a duty to not offer 'stupid and dangerous' opinion.
Amended - It also causes some of the nastiest skin burns I've ever seen.
http://www.emdocs.net/hydrofluoric-acid-burn-keeps-burning/This stuff can kill you and is far worse than any other acid that could be used.