These things are best learned by reading about someone else's mistakes. It can be a bad thing to learn these lessons on your own the hard way.
As stated in many of the posts, the reaction varies from person to person and how moist the skin is. Although 6M H2SO4 did not seem to burn, you can be assured that 12 M or greater will and cause significant damage. H2SO4 is water reactive and generates considerable heat when water is added. A general rule of thumb is for every 10 deg. C increase in temperature, the reaction rate increases by a factor of 2. From my own experience with many years in the lab, HCl will burn or cause pain but inflict the least damage of the common lab acids. You may start to feel a burning sensation at >4 M and definately by 6 or 9M. With HNO3, you will start to get yellow fingers at around 4 M but you may not feel it burn until 8M. At 16 M (70%) it will burn greatly and start to cause significant damage if not washed off quickly. It will have a residual burn for a few hours or longer. At 90% HNO3, immediate blistering, severe damage to the tissues, excruciating pain. This is similar to the effect of 18 M H2SO4 except much quicker. HF, although a weak acid, is the nastiest of them all. Definately never get this one on you. At concentrations around 10%, the onset of pain may be a few hours to 24 hours after exposure. You cannot wash it off at this point. The F- ion migrates through the pores of the skin and enters the blood stream. There it behaves like a strong acid, combines with Calcium (it is a bone seeker), causes excruciating pain for several hours to days, causes permanant tissue damage and often results in tissue death. At concentrations of 49%, immediate blistering of the skin occurs.
I recommend reading medical texts of how these burns were treated. They will make you cringe and give you a lot more respect for those things you work with.
