No, not the type of “hardening” you’re thinking of, moron. If you want that, see my article on kegels.
This article refers to “hardening” in the martial arts sense: Hardening the bones, muscles, and connective tissues (tendons and ligaments) to take tremendous forces for purposes of offense and defense. In other words, body hardening is a family of techniques that will allow you to punch and kick harder, and block harder punches and kicks more effectively.
I specify “family of techniques” because many styles of martial art perform some type of body hardening, and there are many techniques to go about achieving the main goal of making yourself harder. The article here lists two beginner ones, that should be performed before undertaking harder forms of conditioning:
If you will recall, in one of my grip training articles, I discussed how you can do a variation on this kung fu training technique to develop a mighty grip. When done in its traditional way, it also develops the strength of the fingers, hands, and wrists, although not in the sense of being able to generate a lot of force through crushing, punching, or pulling.
No, this technique is specifically for developing the strength of the hands so that they can withstand the tremendous force that will be transmitted through them when the fingers are used in attacks (as is typical of some schools of martial art such as both types of Praying Mantis), typically in attacking the soft tissue of the face/neck and various pressure points.
Before you begin sandblasting, you should develop some modicum of finger strength-I recommend fingertip pushups to do this, and you can read about that here.
You can read about body hardening, with a promise for another article on the subject some time down the road, right here