Went out sketching the other day with the MTL:USK group. Brought two water soluble pens to the Mount Royal Plateau. My new Lamy Safari Extra Fine, and equally new Kuretake #13 brush pen. (Jetpens.com) I swapped out my usual Pentel Pocket Brush, in favor of the Kuretake, to enjoy the water-soluble properties of their cartridges. Here’s some lines, followed by clear water melting. It’s a neat magic trick. World's most convenient watercolor kit.
I can also report, the Kuretake is capable of much finer work with the point (vs. the Pentel GFKP Pocket Brush). It’s about twice the price however, so there is that. As well, the K#13 has a metal barrel, that I find quite slippery, so that’s a bit annoying. But you can’t have everything! You have to love the convenience of a convincing ‘real’ (nylon?) fiber brush in a fountain pen format.