In anticipation of doing washes, took a moment to try wet testing the Carbon ink i got for them with a bit of clear water, there was some transfer for sure, the tip of the brush darkened and the water got a little darker with ink picked up off the lines.
It's not 100% water fast. But it's pretty resilient and lines didn't visibly fuz out.
The shots at the bottom are microscopic photos I took of both lines I did and did not hit with the water.
You can see the subtle shift in tone on the paper from the bit of pigment the water picked up, but also how little the dried carbon ink bleed on the line. Very very little.
It's on the couch on the far right of the drawing where i did it to see the effect at full size.
Oh and the ink/nibs worked very well over correction fluid and bleed proof white, nibs are easy to wipe clean of any white paint so not as damaged as pens can get by it.