Birchwood Casey Perma Fin is a water-based polyurethane resin liquid that is easy to apply and easy to clean. Since it is a single component finish there is no mixing required. Birchwood Casey Perma Fin adheres to metal, plastic and rubber firearm surfaces. You can apply in two ways; apply by airbrush or with a fine-bristled brush. Since Perma Fin is an air cure finish-no baking is required.