I use some homebrew variaion of Red Special my Dad came up with decades ago. The stuff will just about peel the armor off a battleship, but you have to make real sure none of it stays on wood or plastic for very long unless you like spots on your guns.
It's pretty cheap to make, per ounce if you buy the ingredients in large quantities. There's a lot of Red Special variant recipes online to try, I'd recommend trying a few.
