I've had good luck with De Atramentis Document Ink. Dries very quickly, won't smear when I set my beer can^W^W coffee cup down on it. I have been using it for years, and it hasn't screwed up my pen.

I don't know if it is as safe on fine pens as the article would want, but since I use it in a $30 Lamy, I'm not too concerned.

Diamine Registrar Ink is another option. It writes blue then it fades to gray. Some Noodler's (US) inks are resistant to a wide range of solvents as well.

Rohrer und Klingner Dokumentus ink works great too, it's certified to withstand water, organic solvents, bleach, acids and the lot. It bonds with cellulose, but it won't stick to your fingers!

I've found it depends on how soon you want it to be waterproof: I've had ink run when it's got wet within a few weeks of being written, but the exact same ink on the exact same paper (Watermans in a Moleskine) has not smudged a bit when getting damp two years after being written.

Not that there aren't inks that are liable to be much better than that, sooner. But really, if an ink is to flow through the pen, and not have it clog things, I reckon it's unlikely to ever be able to be completely waterproof right after being put on the paper. Unlike a 10¢ ballpoint pen with oil-based ink! :(

Impact paper. It's the future.

What's that?

The future.

Seriously though, I can't find any information for that online. The first Google result is Impact Wholesale, which redirects to some very invasive ads. Impact printers on the other hand are printers that mark dots on the paper using a needle, so no ink is applied.

So, dot matrix printers?

;)

I thought it was "carbonless copy paper", an old tech to replace carbon paper.

https://en.wikipedia.org/wiki/Carbonless_copy_paper