Is the orange peel finish on the bottom, under your paint, from the original coating? If so, sanding on the top won't get rid of it... atleast I'm fairly sure it won't.
I sanded my case with 220 grit to get the finish off, and that was fairly fine paper, and took quite a while (wet sanding). Then I used 600 grit to clean it up a little, and 600 grit between coats of paint.
Of course, that's only if you want a glossy finish. I sanded my case down yet again, using a small orbital sander and 150 grit paper, and repainted it with Tremclad Hammered Silver spray paint. It's a very heavy, thick, paint that works very well, doesn't drip/run, covers up any mistakes very quickly. Unfortunately it takes forever to dry.

When I did the second paint job, I didn't sand in between coats, just in between primer and paint. Infact I only did 1 "coat", layering the paint over and over cause I was fed up at the time it was taking for the paint to dry, and the total time for the case. In the end, it turned out VERY well.