I've been using my pens more for sketching than writing, and I have quite a few go-to sketching tools that I seriously feel lost without. I lean towards using pens with permanent, waterproof ink because I like putting watercolor washes over my sketches.
Last Saturday I bought a bottle of Noodler's X-feather ink, something that's said to look great on any kind of paper. Feathering is a common problem when using fountain pens on paper, and this ink claims to be unaffected by that. It's also anti-forgery, but my signature isn't all that valuable, but okay. Now, X-feather is not at all waterproof, but I think it has its merits, sketching-wise.
After laying down some lines, you can get a waterbrush and put some shadows or grey tones over the sketch. I do this with my non-permanent ink brushpens, like the Akashiya:
and the Paintastics black brush pen, which turns purple when wet:
My other fountain pen inks are too light or too "watery" for sketching purposes, so I found this Noodler's ink really nice. It's good for times when I don't have time to put watercolors, or have no desire to bring colored pencils but still want to use my fountain pens and put some form of "color" in. And because I have a full bottle, I won't be using "I might use up all of my precious permanent ink" as an excuse for not practicing my line work with my fountain pens.
(My friend Ige lets me have some of his Noodler's Black waterproof ink, but I don't want to use up his stash and besides, I've been looking for ink that dilutes to grey this nicely.)
a shoe, a water tumbler, and my pen organizer. quick sketches!
The ink+water combo works best on watercolor paper, but can also be done on plain paper--just expect some bleed-through!