Claire,
Feathering is a nightmare, as you have discovered. I used to blame the paper, but then realized the problem can also be (1) the nib, (2) the ink, and/or (3) or a combination of all of the above. I've used gum sandarac with some success. Sometimes it works, sometimes it doesn't. Can you lay hands on some Pearl-ex powder? This summer, I had to address 150 wedding invitations and the envelopes were the worst I've ever experienced for feathering! Hair spray & gum sandarac had no effect. I tried sealing the envelopes with spray fixative. (It sort-of worked, but made the envelopes "toothy" and "grabby", so they were hard to letter.) I tried acrylic & that worked reasonably well. Finally, I tried Pearl-ex & got no feathers on an untreated envelope. The issue you might face is finding the right color, but if you're using Sumi, black is probably OK & black is available in Pearl-ex. I also go good results from Noodler's Feather-X ink. (It's available on Amazon.) I ended up making a couple of samples for the bride with Noodler's & Pearl-ex & let the bride choose. She liked the Pearl-ex because it is "raised" on the envelope (when you run your finger across it, you can feel the ink) & that was something that was important to her. Try to be patient & work through several solutions, as there is not "one thing" that will work for everyone in every situation.
Send me a personal message if I can help!
Janis
