These water brushes arrive by way of Japan and are quite ingenious. Traveling with a sketchbook, a small pan of cake watercolor and this brush means you are always primed for creativity.
Cake watercolor pans are recommended because the brush picks up color rather than soaks up color. And keeping this in mind it takes very small amounts of water to pick up sufficient medium. It also does away with a container for wash water because the brush is simply cleansed by wiping on a paper towel, napkin or blue jeans. One is cautioned not to squeeze out water to make broad washes for sky or landscape. This is rather a tool for working in close with details.
Designed originally for use with watercolor pencils, it excels in this application. Of great value to the artist is the ability of the brush material to snap back to a sharp point and to maintain this under prolonged use.
The brush of course has limitations but the trade-offs in convenience and availability in the field make up for many of them.

I LOVE these brushes. I've tried a few different brands of waterbrushes, and these are definitely my favorite. The water comes out just right when you squeeze it, unlike some which will squeeze out too little or too much. It's also really easy to refill. And the shape is great because it won't roll off the table. The brush/bristles itself are also the best around. My regular watercolor brushes are pretty much obsolete now because of these Pentel Aquash brushes. The large size is my favorite, but if you're going to be doing extremely detailed work you might want to go for medium or fine.

I love the Pentel Aquash Waterbrush Pen - Large. The large would have to be my favorite out of the small and medium. It can pretty much do both small and mediums jobs in one. I really like the new look it has now. I bought one last year and it was great, but after a while it started leaking and the bush is stained from the paint. So that's when I got another one. I don't know why but this newer one feels different compared to my older one. the water comes out at a steady pace and not all over the place. The cap snaps on and feels like it wont fall of at anytime. The Pentel Aquash Waterbrush Pen - Large is a must buy for when your on the go or just at home. I know I cant live with out it.

Nice, big water brush for large area coverage! This brush is able to spread color in larger spaces with ease. The tip stays nice and pointer, yet the bristles have a good spring to them that helps a round brush this big to retain its shape without wear or damage. What I also like about this brush is that the water reservoir has a fill line so that you don't overfill your pen and cause it to leak—I only just discovered this and am pleased with how it's helped me prevent spillage or unexpected water leaks from the section.

Overall, this water brush is good for bigger paintings than postcards or small water journals, but even if you use those, this brush is versatile enough to create any thick or thin line!

Does not leak!