Nonsparking heavy-cleaning brushes have bronze standard twist-knot bristles to remove paint, rust, deep scratches, and other contaminants from hard metal surfaces. Standard twist-knot wire bristles are flexible and have rope-like, twisted wire along the length for use on irregular surfaces. Bronze bristle brushes are nonsparking for use where flammable vapors and combustible products are present. Bronze is stiffer than brass and can be used at high temperatures. It's often used for cleaning plastic extrusions and creating a satin finish on aluminum. Wheel brushes clean with the edge perpendicular to the workpiece.