Been picking up a handful of these lately. Big fan. Running one with OTR .407 hard to buff and it flows like magic. The nib tends to get over saturated occasionally and drips but the cap doesn't leak. Put OTR 970 paint in another and it does not work. The paint is to thick and it cant flow, even tried roughing the nib up to loosen the fibers but no luck (gunna start messing with the valve system and see if that helps) All the matter pick this up and fill it with ink its legit.
Like this little pen. Filled it with OTR 970 hard to buff ink. Nib doesn't stay super juicy, find myself pumping it a lot but still a great pick up. The refilling issue aint all that big of a deal just get some pliers and lay down some paper to catch any residual ink you're about to spill pulling it apart. Id rather have a tight seal than a leaky one anyways.
I have a few of these, use them as my everyday mop. Have only filled with paint so far (AP MOP 1.0 formula, Junobo and OTR Soultip) no inks. A lot of fun. If you plan on using the 7mm nib be prepared to buy some replacements. Steel roller takes a little getting use to but the longevity is nice. Nibs have the tendency to want to pop out so be careful when opening the cap. All in all great little mop.