Found it. You can use brine{AKA]salt water on your head to toughen it up. As long as you dont over do it and make it crackly lookin. Like a big old thumb callouse. And it sound harsh but also hitting you head with sumthing for about 10 minutes stimulates bruising and helps the muscle strengthen.. Shaulin monks used to bash their head agains walls or boards to do this.
I use a round pipe and just lightly tap it untill to sore to do anymore.