As Greg mentioned, 320 is just a start. It removed the orange peel.
The first two planes I covered in Flite Metal I used the scotch brite pads only. They can be seen on the Flite Metal web site:
http://www.scaleaero.com/kirk_sonniers_p47.htm
The next plane I covered in Flite Metal was after Iread an article in RCJI by Joe Grice were he described wet sanding with 320 and them 400. It was wet sanded with 320 and then 400. That plane can also be seen on the Flite Metal web site:
http://www.scaleaero.com/k_sonnier_p51c.htm
I've also attached a scan of Joe's article from RCJI.
KIrk