sounds like something that might be used in a large industrial setting like large steel tanks or a steel hulled workboat. The second sandblasting being to lightly abrade the surface for mechanical adhesion of the paint. It is not something that should be considered on the outside of a car body that has to look nice.
The outside of your car should be either soda blasted or chemically stripped or rasorbladed down as far as you can go and then sanded to metal. sandblasting does too much damage to do an entire car body. Then follow your paint manafacturers instructions.
