A waterbrush is ideal for using with watercolor paints and watercolor pencils (including lifting color directly from them). Various manufacturers produce waterbrushes, in a few sizes, and in either a round or flat shape. If your local art store doesn't stock them, many online art stores do.
I use a waterbrush for on-site sketching, together with a small travel watercolor set, as it eliminates the need to take a container with water. To clean the brush, I simply squeeze it gently to encourage more water to flow out, then wipe it on a tissue. (Or, I confess, if I've run out of those, on my shirt sleeve.) It doesn't take much water to clean the brush, but it's also easy to refill the waterbrush's reservoir from a tap or a bottle of water.
I've two different brands, and they definitely work slightly differently, with the one having a much easier, continuous flow of water and the other requiring a much more definite squeeze to get water out. I've tried filling my waterbrushes with dilute watercolor and with calligraphy ink, but both clogged up the brush. Again, I think it depends on the brand of your waterbrush (and particle size in the ink) as I've seen a friend use one filled with sepia ink without problems.
I've heard some people say if you're not careful, you can suck paint/water back up into the reservoir from your painting, but this isn't something I've encountered. It may depend on the brand of the waterbrush you're using.
A waterbrush doesn't hold as much pigment as a sable watercolor brush as the bristles as synthetic, so you'll find yourself picking up color more often. The bristles are also prone to staining (as you can see in the photo), but that's hardly unique to a waterbrush.
A waterbrush makes painting from a dark to a light color really simple: you keep painting and the extra water thins the paint until eventually you've got only water. But it also makes painting large areas an even tone trickier than with a conventional brush. However, you'll soon get used to how it works. My travel sketching kit isn't complete without one.
See Also: How to Use a Waterbrush
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