@Baou:
The times Luffy was smacking hody around with haki,it only took like,a second to turn his body parts black.And he switched pretty quickly from arm to leg then back to arm,so I think he has the activation thing down.And Luffy did say,it's very strong for blunt attacks(punches,kicks) but it's still too weak to defend against blades and sharp things(bites).Sonmaybe the vulcanization can be cut by a very strong blade,even though it's hard….
Gaah,the only problem is that's it's inconsistent,that sometimes he turns black,sometimes not...
And i was looking at the panels where Luffy attacked the pacaifista and when he attacked Hody,and the two major difference were:
Against the pacaifista,only his forearm and fist were gear seconded.When he stretches,steam is only coming from the top part .
While against Hody to activate Red Hawk,you see it starts of around his forearm and fist,but the haki(blackness) and steam gradually reach up to his shoulder.So he gear seconded and haki'ed his entire outstretched arm,instead of a small part.
I agree it is a bit inconsitent, but the way I see it is this, Ambition creates an invisible suit of armor. Luffy is known for using his Devil Fruit in creative, unique ways, such as the Gears. It would make perfect sense for him to discover the effect of merging it INTO the rubber itself and using it.
I know where you're coming from; we haven't gotten a solid explanation for Hardening yet. But we've seen him use it Ambition without it, and we've seen him use it. Like I said, maybe it's just the amount of effort needed to be put into it. Luffy did mention when Hody bit him that it hurt him despite him using Ambition, so maybe he didn't use Hardening on his body because he didn't expect the bite to be that powerful.