I remember a while back when Luffy first showed Elephant Gun and a lot of us thought that Luffy had Vulcanized his body to make it black Lol.
Vulcanization
So Vulcanization is a process achieved through very high temperatures and high pressure (also some chemical treatment, but we will just forget about that)
for those who do not know this is the process that is used to make rubber much more durable, harder, and more useful than its raw state. It's how they make the rubber for car tires, soles for shoes, etc.
How and Benefits
Anyway, it is my belief that Luffy will achieve vulcanization through pushing Gear 2nd past its limit. This will result in possible flames, but definitely tremendous heat. Luffy will use this heat to vulcanizatize his body making it tough enough to withstand the heightened Gear 2nd (also giving increased speed), it will make is body harder meaning harder attacks, and the heat from the techniques added with Haki can become explosive
Down Side
This state, by pushing Gear 2 beyond its limit will be exhausting and seeing how hard rubber doesn't stretch as well Luffy may lose his ability to stretch while in this state :|
Vulcanization
So Vulcanization is a process achieved through very high temperatures and high pressure (also some chemical treatment, but we will just forget about that)

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How and Benefits
Anyway, it is my belief that Luffy will achieve vulcanization through pushing Gear 2nd past its limit. This will result in possible flames, but definitely tremendous heat. Luffy will use this heat to vulcanizatize his body making it tough enough to withstand the heightened Gear 2nd (also giving increased speed), it will make is body harder meaning harder attacks, and the heat from the techniques added with Haki can become explosive
Down Side
This state, by pushing Gear 2 beyond its limit will be exhausting and seeing how hard rubber doesn't stretch as well Luffy may lose his ability to stretch while in this state :|