There are several aftermarket coatings available. I have used the black coating sold by Brownells with good results. Note the piece must be sandblasted, or the coating will just flake off. Sandblast, heat with hairdryer until warm to the touch, spray, heat, spray... When you have the number of coats you want, bake in the oven per directions. The bottle says each coat is ~0.003" thick. I did a Cold Steel Scalper, but if you do a folder, it may take some trials to figure out the best thickness.