The basics Acid Brushes are available in two sizes: Medium: 5 3/4 " long x 3/8" diameter.  Large: 5 3/4 " long x 1/2" diameter. The benefits Acid brushes feature black horsehair bristles and strong metal ferrules.   The uses You can use an acid brush to apply glue, or other liquids and [certain] chemicals, and for quick small-area cleaning or wiping applications.