If you are using a D A (dual action air sander) then 80 grit is fine. But using 80 grit by hand will leave scratches in the metal.
If you are doing it all by hand, I would finish with at least 180 grit before primer. If you are using a good 3-part primer, it will fill scratches to a certain extent. But with any primer and paint, it will shrink somewhat and the blemishes will show up later.
Another tip is to wet sand the primer with 320 or 400 grit, and use blocks. Maybe prime and sand 3 times. A good guide is to use a spray can of black paint and just lightly fog over the primer each time. This will show the low and high spots. Wet sand until all the black is gone. If you get bare metal showing in spots, re-prime and do it again and again. Whenever you get the primer slick as glass...you are ready to paint. Remember, paint hides nothing, just makes the blemishes show up more.
