it is an abrasive putty that is pushed through the ports/runners/whatever to remove material. The problem is it removes the material throughout and indiscriminately. When doing proper head porting you don't necessarily want to remove material everywhere and definitely not at the same proportions. My opinion is it is a bad idea in general.
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Don't bother giving me "rep", I won't return it and I think it is stupid.
Gracie Barra Clearwater
Gyms in Clearwater, St. Pete and Port Richey
pm me if you are interested in trying a free class at any of the gyms.