I rub mustard on it, put in zip lock out in sun. (Vinegar) you can use straight vinegar or mustard, ketchup. Either one will do.
If I wanted a more aggressive etch as in real dark, I used to use muratic acid. Don't leave it in there! Just dip and wipe till happy then in to the backing soda solution.
back when we used to eat housecats we marinated them in louisiana hot sauce and soy sauce then season them with lemon pepper........then onto the grill slow cooking them 3 hours........excellent folded in tortillas and pico de gallo----Newgulf