Solution:

A stencil brush is a specialist brush with short, firmly packed bristles. These types of brushes are available in various widths, from small, for tiny, detailed sections, to large ones for faster painting. They’re used in a straight up-and-down pouncing motion, rather than a long swoop side to side or up and down.

The main advantage of a stencil brush over a normal paintbrush is that it reduces the chances of getting paint under the edge of the stencil because of the stiff bristles.