The final sand. It's very important to block polyester primer well with good acrylic blocks. At the end though, with 400 grit, it's also important to be sure that everything is smooth enough for paint (no grooves, lines, hard edges/radiuses, etc). For this, a softer block like these hard foam blocks can be used. Of course still with the proper sanding pattern & blocking sections at a time. But they will help take the edge off of things before paint.