Hydrofluoric acid is a weak acid used primarily in industrial applications for glass etching and metal cleaning/plating. It is contained in home rust removers. Although technically a weak acid, it is very dangerous and burns can be subtle in appearance while having severe consequences.

Wilkes G. Hydrofluoric Acid Burns. Jan 28, 2010. 
http://emedicine.medscape.com/article/773304-overview

• 2 mechanisms that cause tissue damage*
  • corrosive burn from the free hydrogen ions
  • chemical burn from tissue penetration of the fluoride ions
• Clinical features*
  • Cutaneous burns - absent findings to white-blue appearance
  • Pulmonary edema
  • Hypocalcemia, hyperkalemia, hypomagnesemia
• Treatment*
  • Decontaminate by irrigation with copious amounts of water.
  • With any evidence of hypocalcemia, immediately administer 10% calcium gluconate IV.
  • Cutaneous burns:
    • Apply 2.5% calcium gluconate gel to the affected area. If the proprietary gel is not available, constitute by dissolving 10% calcium gluconate solution in 3 times the volume of a water-soluble lubricant (eg, KY gel). For burns to the fingers, retain gel in a latex glove.
    • If pain persists for more than 30 minutes after application of calcium gluconate gel, further treatment is required. Subcutaneous infiltration of calcium gluconate is recommended at a dose of 0.5 mL of a 5% solution per square centimeter of surface burn extending 0.5 cm beyond the margin of involved tissue (10% calcium gluconate solution can be irritating to the tissue).
      • Do not use the chloride salt because it is an irritant and may cause tissue damage.

*Extracted from emedicine article.

References

Wilkes G. Hydrofluoric Acid Burns. Jan 28, 2010. 
http://emedicine.medscape.com/article/773304-overview