A dosa sizing brush is a brush that is used to apply dosa (sizing) when preparing for and working on a Japanese painting. It is mainly used for sizing washi and silk canvas so that the ink does not run, and for gilding with gold or silver leaf.

The width of dosa brushes varies from around 3cm to around 21cm. Generally, the brush is made of pure sheep wool, which is soft and has good composition, and has an unpainted wooden handle coated with water-resistant lacquer at the edges. This lacquer finish prevents the handle from cracking when rinsing off alum with hot water. The prerequisite of a good bush is that it has good liquid composition and coverage. When loaded with plenty of liquid, the brush tip should not split, allowing the liquid to be applied evenly.

When sizing, an even concentration of sizing liquid should be applied, ensuring pools and streaks are not produced with the brush strokes. Load the brush with plenty of sizing liquid, lay the brush flat to begin with and gradually raise it upright while move it sideways. This way, a uniform amount can be applied. Ensure that the brush strokes do not overlap too much, and also that there are no gaps left behind. If the sizing solution is applied unevenly, this could cause paint to smudge and cause variations in color. When gilding with gold and silver leaf, sizing liquid or glue is used to fix the leaf on to the work. Moreover, applying sizing solution over the leaf helps fix paint to the surface and also prevents oxidation of the silver leaf and silver pigments.

Various types of brushes are used in Japanese paintings depending on their use. For example, besides the dosa sizing brush, there is also wetting brush (mizubake), painting brush (ebake), and shading brush (karabake ), etc. These should always be used only for their respective purpose because once used, they will each have different substances and paint residue on them. The alum in the sizing liquid repels paint so using the same brush would hinder the spread of the paint. When a dosa sizing brush is used for a long time, the alum in the sizing solution can degrade the brush, causing the brush tip to split. After use, wash the brush thoroughly in cold water and then with lukewarm water to remove the sizing solution completely, drain excess moisture, and hang to dry in a well-ventilated location not exposed to direct sunlight. Dosa sizing brushes can be purchased at art stores selling Japanese art supplies.