I actually bought an extra robe for Kylo (because I wanted the scarf and it was attached) - because I messed up the scarf trying to customize mine lol.
I did the water treatment on one of them by soaking it, and the other by spraying. The results were similar, at least in my experience. Though, I would note that I took the robes off before doing the treatment, so I didn't dunk the figure as a whole.
The only thing I would try NOT to dunk is the scarf if you still have it attached. There are a few spots where the scarf is held together by glue and it can easily come apart if it's soaked.
He may be a ***** to futz, but Kylo is definitely my favorite figure I own.. such a cool piece to look at, fun to pose, and the textures and sculpt look amazing IMO.
I have mine with his hood down in what I like to imagine is a "Kylo in the rain with the Knights of Ren" pose.