The procedure at our shop is to first test a sliver of the material to see if it floats or sinks. You have to be sure to knock off any air bubbles that might be making the piece float, but if the sliver floats, the material needs adpro. So we would scrub plastic that needs adpro with gold scotchbrite and Dawn soap, then clean with #700. After that, adpro and then either sealer or primer are applied. Plastics that don't need adpro tend to sand better, so they can be sanded with regular old 320 and red scotchbrite, cleaned with #700, and primed/sealed directly. The reason we use extra fine scuff pads on the plastics that need adpro is because they get all "hairy" and look and sand like hell if they've been sanded with coarser pads or grits. But if you are going to primer the part twice, that probably doesn't matter.
