lklawson
Grandmaster
It doesn't cause permanent injury, provided the tanic acid is sufficiently week. About the same roughly as vinegar or urine.I wouldn't count discoloration as an injury. Dermititis would be. I wonder what level of exposure that would take. At a tannery, they'd be in contact with it all day if they didn't use PPE.
You're not trying to get an acid burn on your skin. You're trying to promote a chemical change in the upper layers which makes the skin thicker and less elastic. As long as you don't die or damage the epidermis beyond its ability to heal, then your skill will grow. It's a little like what happens to callouses which aren't "maintained."
Peace favor your sword,
Kirk