Sanding/Painting advise needed

[gc]

If you need to paint a project (redwood in undercoat/eggshell) what grit of sandpaper would you go down to.
Sanded a bit last night to 320 grit but wondered if the wood would need more of a key from a lower grit to help paint?

 

[cornucopia]

320 is a bit fine- i would prep with 120 or 180 and de-nib between coats with 240 or 320