I stopped by my local hardware store a few days ago looking for silicone grease as used by OfficeSupplyGeek to make his own highlighters. They had never heard of the grease, so I picked up a can of Pro Dope pipe joint compound, a gray goo used to seal pipe threads. I wanted to give this a try in addition to the Teflon tape I've used to seal pens. The process is simple: brush the compound (there is a brush attached to the lid) around the threads, then twist the pieces together until tight. Wipe away excess compound, and your pen is now water-tight and leak-proof.
The pen I'm using is a Platinum Preppy Sign felt tip I got at Kinokuniya just for this purpose. The line is a touch thicker than I normally like for writing, but life is not meant to be lived in fine point alone. The ink saturates well, and the pen flows smoothly over the Rhodia 80 g/m paper. The color is a mix of Private Reserve Chocolate and J. Herbin Bouguet D'Antan, a pale rose. Like the Diabolo Menthe, the Bouguet D'Antan is too light for me to use for writing, so I decided to try for a brown-pink ink. The mix is about 1 part Chocolate to 2 parts Bouquet D'Antan, and is a nice dark warm brown with a hint of rose.
UPDATE: Since making this felt tip a few days ago I was trying out something and rinsed off the marker. The brown is now less intense and the rose-pink more noticeable. I like both colors, so whichever it stays or reverts to is fine with me. The only "problem" is that ink has gotten into the threads near the tip of the marker--that part does not come out so I couldn't put any compound there. Nothing has leaked out of the front of the marker, and the brown ink in the threads actually gives the pen a nice look; I'm just wondering what will happen later.