I'm no expert but my research on prepping surfaces for airbrushing (yes, I'm getting slightly into airbrushing) would be to start with a 600 to 800 for the first sanding if the surface is in good shape. Go with finer grit at each coat while wet sanding.
Check airbrushing forums. A good one is airbrush.com but do use the search to not irritate the folks there.
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Louis
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