In article .com,
wrote:

I find that sandpaper between 100 and 200 grit works great for sanding
down brushes to fit. Wear rubber gloves if possible, and plan on
throwing the sandpaper away even if it is hardly worn. The black
carbon gets into EVERYTHING!.

Whether you use a file or sandpaper, do the shaping under a stream of
running water to avoid a terrific mess. Wet-or-dry carborundum paper is
your friend.

Isaac