As a calligrapher, I have to work with all kinds of writing fluids and surfaces. For making French lines and washed panels with inks, sometimes it is appropriate to treat the surface of matboard by gently rubbing sandarac into it. Sandarac is a powdered gum that improves the puddling of fluids. It seems something like powdered shellac.
Most matboard surfaces -- including all of the Rag Mats, come with a very good surface for gouache (aka opaque watercolor), the most forgiving fluid for most paper surfaces. Like paint, gouache lays on top of the paper surface, whereas inks and ordinary watercolors soak in. So, if you make a slight over-run of a line with gouache, you can scrape it off the board with a razor blade. On the other hand, ink penetrates, so it's in the paper to stay.
Intermixable gouache colors have a consistency like toothpaste and come in tubes. Dilute the paste with filtered water to the consistency of buttermilk.
To improve flow, add ox gall, which is a wetting agent. In some cases that will make it bleed. To improve puddling and avoid bleeding, add gum arabic (I prefer it powdered), which is the primary binder in most watercolors. Either way, add only a slight amount at a time. That is, add what you get by dipping 1/16" of a thin stirring stick into it. A little goes a long way.
If you want to have some fun with a co-worker, add gross amounts of both ox gall and gum arabic to some gouache. That would make it very difficult to flow out of the pen, and when it comes out it will bleed uncontrollably. That's what calligraphers call fun, folks.
Acrylic gouache, aka airbrush color, also works well. However, the additives described above will not mix with it. Use the liquid as it comes in the bottle, or add only filtered water if necessary. In pens and brushes, acrylic gouache handles a lot like traditional gouache, but it is not water soluble after drying. That characteristic makes it ideal for drawing pen lines to contain washed-color panels. It will not bleed if re-wetted, as traditional gouache can.
When I make mat with washed-color panels, I first outline them with ruling-pen lines of acrylic gouach. Then I apply plain water to wet the paper surface, which makes blending of the colors much easier. By this technique you can make a color diminish very gradually. For washes, I generally use traditional gouache with lots of water -- and only water -- added.
