On Thu, 17 Mar 2016 10:56:37 -0700, DaveC wrote:

Choose one of these, try acetone on a Q-tip cotton ear bud to dissolve
the coating (try to not disturb the €œthing€� that lies below the
coating--if,
in fact it is below the coating). Get a €œburnishing brush€� with
fiberglass bristles (used to clean oxidation from copper, etc.) and
start gently rubbing the site. I think youll quickly discover if its a
surface growth, or a crack or other phenomenon.

Good luck.

Thanks. They're definitely not cracks, they're fibres of some sort.