You should never use a metal wire brush near the center electrode insulator. If its surface is not perfectly smooth, it will abrade the metal bristles, leaving a conductive track on the insulator. Many surfaces and all ceramics are harder than common metals, and will present this problem, particularly if they are supposed to be an insulator.

Steve

__________________
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7