Oh, no. He's definitely used it before and he probably used it in all 3 of those encounters but I was referring to the hardening where the armament haki is visible. Perhaps it's not necessarily stronger than the invisible one but I'd like to see Sanji utilize armament haki more often and that it'd be nice to see it since it's kinda unclear when he's using it (unless he's just constantly using it and there's no difference).
