I clean white china bristle brushes using paint thinner and one of
those metal combs used for cleaning brushes. This appears to work but
after time the amount of "usable" bristles gets smaller. From the
metal piece growning downward it turns into a rock.

I purposely try to get this section by turning the brush upside down
with fresh paint thinner on it and working it with the comb.

What am I doing wrong, or is this just life? Thanks.