This page includes a couple of art supply sketches and a test of my fountain pen ink when water is applied. It turns out most fountain pen ink is not waterproof, or even water resistant, because properties that make an ink waterproof also make it likely to clog or even ruin a fountain pen. You can get a nice fountain pen for $25, but when some cost upwards or $300, I want to be sure I know a little about them.
I ordered Noodlers Heart of Darkness ink for my fountain pen because so many people who enjoy drawing with a fountain pen swear by it. I I like to watercolor over my ink so tested for waterproof properties. (only on two samples of paper). I found the HOD to be mostly water resistant on writing paper almost immediately, but I guess it just sits on the surface of this particular watercolor paper, because even 24 hours later it still bled. A lot. Disappointing. I’ll try it on another paper. Meanwhile, I’ve ordered another ink that I saw recommended by a youtube artist I follow.