you shouldnt sand past 200grit for a primer coat.

The paint needs some "teeth" to adhere to.

I find that the clear poly I use sticks better and lasts longer when I sand with 60grit only. It gives the finish somethign to grab onto. This varies by paint type, but I never sand past 200grit before applying top coats for any paint type.

Once the top coat is on, then you can sand that with 600, 1000, etc to get a mirror finish.

Bill