Just a couple of thoughts on the matter- - - - - -when I ran a body shop, I would absolutely refuse to paint a vehicle with somebody else's prime job on it. The only exception to that rule would be if I removed all the primer to bare metal and started over. Primer is the base that all the rest of the paint job uses to hang onto the metal. Any mistake or application error will trash the paint job before you start.
Sandblasted sheet metal will have ripples in the grain structure of the metal that cannot be sanded smooth- - - -don't bother trying. Multiple layers of a high-build primer such as self-etching epoxy might cover up the ripples, but you'll need to prime, then sand, prime again, sand with a finer grit of paper, and prime and sand at least one more time. I would start with 180 grit, then 220, and maybe 320 before applying the color coat. Once you start priming, don't let the primer get exposed to weather- - - it's porous, and soaks up water like a sponge. If you drive a primed vehicle and it rains, the absorbed rainwater is trapped under the paint when you get around to applying the color coat. Eventually that moisture results in rust under the paint, or bubbles from poor adhesion of the color coat. Before applying the first coat of primer, spend a LOT of time with a high-pressure air hose, blowing as much of the sand residue off the metal as possible. A less aggressive blasting media such as glass beads or baking soda does a lot less damage to the metal. NEVER use sand on sheet metal.
Jerry
