Catalyst Polytip Filbert 6 Short Handle This short handled Catalyst Polytip Bristle brush by Princeton features a size 6 filbert head. Catalyst brushes are designed with a stiff yet still extremely responsive synthetic hair that has real integrity in water. Each individual hair has been split to replicate the natural flags on the finest natural bristle. Designed for use with medium to heavy-bodied acrylics and oil paints, catalyst is able to hold a higher volume of paint while providing a smoother application.