Hello,

Two observations from someone who was in research with Olin Hunt when we
developed Black Hole;
 1. Possibility that your etch filtration system on the Black Hole line needs
upgrading. If you filter the carbon out it won't redeposit. As you know any
redeposition is as hard as a rock!
 2. If that does not do the trick try an innerlayer scrub brush ( Lippert
abrasives makes them ). They don't reach down into the holes like bristles
do.

Feel free to call: Mike Walsh 800 645 2996 x 269