I’ve lost track of how many inks I’ve tried that a) won’t clog up a fountain pen
b) write/draw smoothly and
c) can cope with a watercolour wash
I’VE FOUND IT!
Behold De Atramentis Document Ink in black (bought from purepens.com)
Barely even needs 10 seconds of dry time before it can cope with water. Pen is Pilot EF, so I’ll try it with a wetter feed pen in future, but so far am loving this combo.