Brian 
The Pentel Pocket Brush Pen has been one of those tools that has been at my side consistently since I first bought it. Given the recent world-shattering news–okay, granted, it’s a rather small world– that sable hair brushes have now been banned from the US, artists are going to be taking extra special care of the brushes they have while looking for alternatives. I’ve found the Pentel Pocket Brush fills in nicely for inking. If you’re lamenting the demise of the sable brushes for watercolors…well, let me know when you find a good alternative.
It’s not exactly the same as a good Windsor Newton Series 7 or a Raphael, but what it does it does well. I know some artists have a hard time making the switch, mostly because of the little “tail” at the end of the brush. Granted, that does take some getting used to. But once you do, there is a joy unique to this brush as you watch it dance across the page. Enough practice and you can get some seriously tiny detail with it too. Again, if you can get used to it. I fully understand how some people might not get the hang of it, or want to. And it’s not the creamy smooth pull of a sable brush when the ink is right.
The pens come with cartridges, but honestly I’ve found it more cost-effective (BY FAR) to buy Rapidiograph Ultradraw ink and refill them. The ink is nice and black and doesn’t gum up the bristles. Never ever EVER use India ink in it. That will kill it. Trust me. Also, never leave it laying around when the cable guy comes over or it may get stolen. But that’s probably just that one time.
Anyway, I’m inking quite a bit of GI Joe: Real American Hero with it. Here’s a few pages from issue 205. 
(Click for a larger view.)
You can buy both the ink and the brush at the links below.
http://www.dickblick.com/products/pentel-pocket-brush-pen/
http://www.dickblick.com/products/koh-i-noor-rapidograph-ultradraw-waterproof-ink/
Nice to see someone has experimented using alternative inks for the PB. What other inks have you used (if any)? Would any rapidograph ink work (here you can hardly get even rotring)? Also, one of the cool things about the original FP10 ink is that it dries waterproof and flows of the brush smoothly, does the ink you use work the same?
Sorry for bothering you with son many questions and thanks for your time!
I tried using india ink, but it gummed up the brush to bad (in fact, I had to throw that brush away.) The Rapidograph Ultradraw is the only other one I’ve used and it dried very dark, is waterproof and a bottle of it will keep the brush filled for a long time.
Hello, I found your article very helpful but I have question:
You said to never use India ink, but in the specifications from the website it says it is a India ink. So what’s the difference?
Is this a lighter ink? I just got the brush pen I don’t want to ruin it.
Thank you
It’s okay to use the Ultradraw. The Ultradraw ink is made for Rapidograph pens and I’ve been using it for years with no problem. Just don’t use something like Speedball ink or an india ink that’s made for use with brushes. The Ultradraw doesn’t clog/gum up like the others. Sorry I can’t be more science-y as to why, I just know it works–despite what’s on the label 🙂
Monica, the issue is probably due to the high level of lacquer that inks like Speedball use. it’s what makes them so glossy and opaque, but also dries rigid and can force the bristles of your regular brush apart, rot the hairs, and get clogged in the pocket brush nozzle thingy.
I’ve only ever heard of people dipping their brush pens in speedball, never filling the cartridge, for good reason.
I’m debating whether to try rapidograph universal india in mine, everything about the brush is synthetic so it’s inert to ammonia for cleaning out any possible clogs. I’ve been refilling with carbon black inks with no issues so far but it’s not as rich as shellac/laquer inks.
I’ll definitely pick up a bottle of ultradraw on your recommendation, thanks Brian.
What’s the difference on paper between rapidographs universal india and ultradraw? which is darker?
I’ve never used the Universal India, but the ultradraw is jet black. When I dip a brush into speedball ink (which is rarely black these days) and use it alongside the Ultradraw there’s a noticeable different. The ultradraw makes for a nice, smooth black fill.