@Darkstorm:
Not that strong.
It's been mentioned that there's a specific kanji for vulcanisation that wasn't used here.
Plus of course vulcanisation being a one way process and all.
Unless somone like aohige or stephen has confirmed and I missed it?
By wikipedia (a mixture of Japanese and English). The exact kanji are used for the "curing" of substances, which is also called vulcanization when it applies to rubber. Ridiculously strong evidence; no, it is not an exact translation, however, based on the aesthetics, I think it's the most faithful. An exact translation would, instead, be "curing." That doesn't sound threatening.
http://ja.wikipedia.org/wiki/%E7%A1%AC%E5%8C%96
Click the English button in the bottom left if you want proof. This was originally proposed by Kiavik, and quite frankly, he's OBVIOUSLY correct.
Also, for people who care THAT MUCH about consistency, I think I have an answer about Luffy's underwater Gear3:
He's beginning to gain control of his nervous system to the extent of Mr. Fantastic, such that he is literally in charge of how much individual parts of his body are constricting or not (whereas before, he had to force his bones to stretch). The fact that Gear2 is even possible is further evidence of this approach.
Also, it would trivially follow the already-proven-possible-in-this-universe method of fighting utilizing Biofeedback, such as Kumadori using his hair. The reason he still needs to blow air into his thumb at ALL is because that makes it easier to get the limb expansion started in the first place, using the principles behind inertia. Without haki, this form wouldn't be maintainable long enough to land a punch, but if he instantly vulcanizes his fist at its peak expansion (despite it being mostly vacuum), he can maintain the form indefinitely.
Then, when he lets Gear3 off, he again uses a basic level of biofeedback to force his form to stay balanced at normal size, instead of going chibi.
But again, Oda probably doesn't care. However, I believe this answer is consistent with what we know about the OP world.