Similar to a felt-tip pen, its business end is a bundle of glass fibers that are just hard enough to be used as an abrasive on paint, and lightly on metal. In fact, the fibers' softness creates a self-regulating effect. When the roughness is gone and the nicked area smooth, force is distributed evenly across the many bristles and cutting slows appreciably. The tip's narrowness allows you to clean and smooth a repair area without appreciably enlarging it.
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