It seems the road to CoC and CoC coating is different to each character. I always assumed that Haki to be a regid power system where you advance from lvl to lvl through experience and power lvl, this is partially true for CoA, but for CoO and especially CoC every character has his own hurdles and personality flaws to overcome to advance, case in point Zoro needed to consciously project his sense of self into his sword to advance his CoC while Sanji needed to expand his love inward to advance his CoC.
While Luffy, with his simple character, he didn't face major hurdles to advance his CoC and Haki in general.
In conclusion CoC in particular seems to be a character driven power system. The better you become at understanding your inner self the stronger and more skilled you become.