You can get fountain pen inks that chemically react with cellulose for complete water resistance (there may still be a little smudging from unreacted ink washing off the paper). Noodler's Black is a famous example. I like this kind of ink, although it's not without its drawbacks, because cotton/rayon/other plant fiber clothing is also mostly cellulose and will be stained just as permanently.
I've used only Noodler's Bulletproof Black for years (in a TWSBI Eco piston-fill) - it works great, and I realized if I was going to take the time to write extensive notes it was worth having some protection against water.
I previously used Waterman Black - it flows really smoothly, but it isn't waterproof. Also, it doesn't seem quite as dark as the Noodler's.
> inks that chemically react with cellulose
I had no idea. Great tip; thanks.
I'm looking for alternatives to lamination for posting outdoor flyers & signs.
I just received some (alleged) weatherproof copier paper to experiment with.
https://www.amazon.com/dp/B01MQ3UNWL