Today NONE of my white inks wanted to do the job! My McCaffery's ivory was too faint (the French postal workers won't try very hard to read faint hairlines, I find!), and my NEW bottle of Pen-White would not cooperate at all. It kept pooling at the baseline! Annoying, and gum arabic wasn't helping me at all.
I got up, stomped over to the sink and quickly mixed up a bit of Linel Gouache, making quite runny whole-milk consistency and filling a dappen dish. I added a drop of gum arabic to the mix and VOILÀ ! Why do I ever bother with white ink when gouache is so easy? Sometimes I want the convenience straight out of the bottle, but getting familiar with gouache is a good idea if you're not already. Comes in so handy when you need to color-match when addressing envelopes, too...