I need to know if this is true or not!! If it is then I will not use this technique for my port and polish. It is good to learn from others mistakes. I am going to go another route. I have a bag of playground sand and a shop vac. I'm not going to run the engine, like a sucker!! I'm going to leave the engine off and hook the shop vac up to the exhaust pipe and suck the sand through the airbox, thus keeping the sand from scratching up my cylinder walls.