Hi Michael. I'm just seeing this post and was wondering how your journey with mixing gouache is going? I'm not exactly a beginner but not exactly intermediate either. This year, I used a gouache mixed with water for calligraphing my Christmas envelopes. It worked quite nicely, but something I didn't think about at first was that I felt the necessity to seal the envelopes. I feel like the gouache mixture would bleed very readily even if a mist hit it. Whereas sumi inks - although not waterproof - might withstand this easier. Dr. Phil Martin's Bleedproof White - again, not waterproof - would also hold up to a drop or two of water without completely messing it up. But not the gouache.
If you like using the gouache and you want to use it for addressing and envelope, I had very good luck with this product: Gold Matte Archival Varnish. It is a spray and you have to shake the can for 2 minutes before use. Start spraying slightly above where you want the spray to hit and move from side to side - about 7 - 12 inches away. I felt much safer sending out my gouache-inked envelopes after doing this. If someone had so much as touched it with a wet finger, it would've smeared - even after being dry.