Brushes with long hairs will hold more paint but that also makes them softer. In the picture below you see examples of brushes with hair about 1 inch or longer. At the top is a Bill Alexander liner (a.k.a. script) brush that is like a long hair round made of red sable. Below that are two brushes from Andrew Mack. This brand is typically sold to sign painters, so you’ll have to look around for them in specialty shops or online. The short handled one is a quill brush, and the other a one-stroke. One-strokes are also sometimes called lettering or show-card brushes. The bottom two are from Loew-Cornell. The LC American Painter (gray handle) is labeled a “flat” but has a longer hair length than what is typical. The bottom LC brush is a “7100 Stroke” size ½. These LC brushes have taklon (polyester) hairs. The Mack one-stroke is squirrel and ox hair, and the quill is all squirrel.
These brushes are particularly useful with gouache paint, since it has a fairly weak binder, so the longer hairs will be softer and the narrow width makes them lighter. Those features make them less likely to disturb the paint underneath for over painting or glazing. You’ll still need a light touch, but they handle better in that application. Round hair watercolor brushes may be soft also, but the width of their belly makes them heavier.
They also work well with watercolor, casein, or egg tempera. You’ll get best results with paint that is fairly fluid. I find these extremely useful for layering casein. For a long time I had heard to not use natural hair brushes with casein since the alkali might frizz them out, but I’ve never noticed that problem. I’d be more worried about using acrylics with these, except for maybe the taklon. Mack makes brushes that are blends of taklon and squirrel hair that would work better for acrylics, and oils too I’d imagine, but I typically don’t need extra soft brushes for those mediums anyway. There is also a type of brush called an “egbert” that has a “c” shaped tip like a “filbert” but with longer hair. Where did they get those names I wonder?
One Stroke brush
By manipulating the angle you can get a variety of lines, and their length adds more flexibility. In the example above you can see a comparison between marks made with the Mack one-stroke and a Winsor & Newton Cotman round #4 brush that has about the same width. The image on the left above shows blue gouache made with the one-stroke, and after that dried was painted a fairly wet transparent yellow. Notice how the blue is not disturbed. Compare that to the image on the right where the same yellow was applied with the Cotman watercolor brush. The shape of the round brush causes the paint to plow up on the outer edges and the belly to drag into the paint below. The point of the round can be useful, but the shape can cause problems in this application.