After being a Flat rate guy for over 30 years I can say this. Yes if you're not very careful you can get enough grit in a motor to cause damage. Yes if you hold a disc in one spot long enough it's not going to be flat. Small diameter discs and worn discs make the problem worse. I never had an intake or head gasket comeback. As a flat rate tech it always pays worse when doing the the job a second time for free. If they didn't work I would have never used them. That being said, inexperienced hands can do a lot of damage in a hurry. Now a blue printed performance engine, different story. Not even the same animal.
Doug
