I'm sorry to tell you all, but most of you are just flat wrong. Not only is this historically accurate but is a long standing tradition and it worked.
Using brine is one component of a traditional method of skin toughening from the European tradition. It's similar to some Asian body toughening techniques, often accompanied with Jow.
Most of the information comes from 19th Century pugilism. The knuckles/fists are usually intended target for toughening, though the face is sometimes also. Usually there is some regiment of bag punching and then an application of a solution or tincture. Besides a (salt pork barrel) brine solution, various tanic acid applications were used. Dempsey recommended the application of a product called "camphor ice" which I've not been able to find any other reference to. Another period author referred to a commercial tincture he called "Tom Sayer's brew" which included "turps" and some unknown herbs. Some authors apparently suggested using rosin and there's a rumor floating around that some authors might have recommend the use of urine, though I have not found a source for this. The boxer "Ruby Red" Bob Fitzsimmons recommended a solution which included laudanum.
The brining and acid applications (tanic acid & urine) would help to cross-link the collagen in the skin which would toughen it up by maintaining the flexibility but reducing the elasticity.
Keith P. Myers did a lot of research on this.
Peace favor your sword,
Kirk