An oval sash brush is shaped similar to an round artists brush only crimped flatter so it is wider not round. It is designed to work with the intricate designs in carvings, scrolls and wrought iron railings normally with oil based paints. It features a tapered handle design.
Brushes constructed with natural bristle, the best of which comes from China, should only be used for applying oil-based paints or for oil based clear finishes such as varnishes. Avoid using natural bristle with latex-based or water thinned paint products because it will absorb up to 40% of its own weight in water, causing it to be too soft to paint well. Rough surfaces break the tips off natural bristle so it will not produce a smooth glass like finish. Nylon is five times more durable than bristle so use a synthetic for painting rough surfaces, even with oil based paints.
Black bristle is slightly stiffer than white bristle. The tips have thicker flags than white bristle. It is excellent for high productivity in oil based interior and exterior applications. How To Choose The Right Paint Brush For The Job