David
Spark plug efficiency degrades rapidly as the corner of the electrode is rounded off. For the best efficiency, if you want to sand blast plugs for your "POS race car, lawn mower or Toyota", I would suggest filing the top of the electrode after cleaning.
I had just assumed you were blasting plugs clean for your Cobra. I also agree with the other regarding running overly rich.
http://www.ngksparkplugs.com/techinf...q/faqread2.asp