Dana, this subject comes up often and usually turns into a major peeing match. I rather it happen here than on a thread of yours in the future titled "Sand blasting ruined my car, what do I do?"
"Sand blasting" can DESTROY sheetmetal panels. I don't mean "damage" them, I don't mean "Darn it, I messed up my hood" sort of damage. I mean "&^%$# I RUINED MY HOOD AND HAVE TO THROW IT AWAY" sort of damage.
Unless who you have doing it REALLY knows how to strip the paint from a CAR, not a dump truck, not a tractor, but SHEETMETAL on a cars body, DON'T DO IT!
So, let's start with that, you have your answer to the primer, that is the easy one. Let's delve into the whole reason you are sandblasting and how it is being done.
Brian