I would still use a "prep" to get rid of the fine dust/grit from the sand blasting and compressed air is not always free of contaminates from rust (condensation/humidity) or even oil in the compressor tank. If your compressor has a water separator or is set up properly to spray paint then that would take care of clean, dry air but still the dust and grit from sand blasting will still be there DAMHIK. I would prep them just the same way as if you were painting a body panel.......just me though.