Flat, natural bristle brush used for applying stiffeners and primers.
Natural bristle brushes work the best because the bristles of the brushes will not curl. The brush does not absorb too much product so that it can be applied economically. They can also be left to dry without cleaning, then softened for use again by resoaking in the product. This conserves your stiffener.
It is best to keep a brush to use with each concentration of stiffener.