A sash and trim brush normally comes with a thin and long handle to aid in maneuvering. A small, square-edged or flat sash brush works best for painting thin objects such as the thin pieces of wood that separate individual panes of glass in doors or windows. Straight sash brushes vary in width from 1 inch to 3 inch with 1-1/2 and 2 inch being the most popular. Professionals may opt for larger sizes. Straight flat sash brushes come with a slim long handle such as a rat-tail or pencil handle.
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.
White bristle is softer and more flexible that black bristle. It has thinner feather like flags on the tips. It is excellent for fine finishing on the interior with oil enamels, urethane and varnishes. It is also used for fine finishing on exterior high gloss smooth surfaces.
Gray bristle is a cost effective blending of bristles that are unsorted and is fine for maintenance grade utility work with oil and rust resistant primers and enamels. It is softer than black bristle, and stiffer than white bristle. How To Choose The Right Paint Brush For The Job