For jobs that require extra strength, the newest and toughest of the Ultra/Pro line is Extra/Firm. NylonPlus filament, exclusive to Wooster, allows for superior stiffness and cut-in control. Able to maintain sharp edge even in hot, humid conditions. Made from 100% nylon. Very durable; cleans much more easily than brushes that use polyester to achieve extra stiffness. Square steel ferrule, chisel trim.