Sanding tips for spray-paint over poly?

[zebra]

I'd got a old guitar with a poly finish that I'm planning on spray-painting.  It's an older guitar that's been beaten up a bit and is chippy, so I'm planning on sanding it down a bit to even things out.  Deep enough to remove the top color coats, but not so deep to remove the clear base coat.  (It's a crappy plywood body - otherwise I'd go down to the wood and finish with tung oil.)

After evening out the surface with a more aggressive grit, how fine a grit should I go until it's ready to spray? 
Seems like it should be smooth enough to allow for consistent coating, but not so smooth that the paint doesn't adhere well.

I found a couple other threads about the spray-painting itself, so I feel ok about that part.
But if anyone wants to throw in so extra hints, I'd appreciate it!

 

[Tonar8352]

I final sand with 320 before starting to spray and also use 320 for between coat level sanding.

 

[NedRyerson]

Out of curiosity, would there be any chemical compatibility issues with paint over poly?  Just want to take mental notes in case I do this myself in the future.

 

[mayfly]

I don't think so - Poly is pretty inert stuff.  My main worry would be getting whatever you're using to stick to it