HF is evil, evil, vicious fucking lethal stuff.
Due to the incredible electronegativity of fluorine and the resulting way the bond energy of the F-H and F-C bond is so great, fluorine-other element bonds are very tight and hard to disrupt, HF is actually a weak acid, the weakest of all the hydrohalic acids, thanks to the dissociation constant being so very low, as the hydrofluoric acid can't release its proton save with great difficulty and thus sinks straight through the skin to weak its havoc.
Not only does it then go on to dissolve one's bones from the inside out, sequestering calcium from tissue, bone and blood, causing massive hypocalcaemia and subsequent toxicity from the fluoride ion itself (fluoride is nephrotoxic, attacking the kidneys); the hypocalcaemia affecting heart rhythm. HF burns from weak HF are often, as you said, not painful until later, and not immediately obvious, unlike conc. HF burns, which are apparently absolutely agonizingly painful. Said to be like whacking your (name bodypart(s) here)
