Brush pens can have a wide range of results when exposed to water. Some brush pens are absolutely waterproof, the second the ink touches the paper it quickly dries and becomes permanent. If you draw a line with such a pen and brush water over it you will see no change. Other brush pens are partially waterproof. When exposed to water, they may run but keep their overall color and outline. Lastly, there are brushes that are water-soluble. These brushes act like the black color from a watercolor set. You can mix them with other colors to create darker shades, etc.
Each brush is unique, and its special characteristics will create different effects in your artwork. For example, an absolutely waterproof brush is ideal for outlining, whereas a more water-soluble brush is great for blending. You'll want to pick a brush based upon your desired affect. The following chart shows the waterproofness of some of our most popular brush pens. The right side of each box shows the result after being brushed with a Aquash water brush. The left side was left untouched.