User blog comment:Black Leg Sanji/Time Skip Power/@comment-1595565-20100912152642/@comment-2031609-20100913161050

I too can only see Sanji, Zoro, Robin and maybe Franky possibly learning haki, for pretty much the same reasoning. Sanji and Zoro are going to be fighting/training for the whole 2 years, and who knows what'll happen when it comes to Dragon or Vegapunk. Usopp might catch a glimps of it, but he won't be able to really use it. But I do think it would be cool if either Franky or Usopp could learn to infuse haki into weapons at some point in the future.