Finish exterior bodywork, apply first prime coat.
If there is a headliner, mask exterior; spray headliner in flat color.
Wet-sand and prime exterior final coats after first masking inside body.
Prep & spray all exterior.
Once fully cured, use medium size brush to paint inside body flat black. The idea is to "hide" things like inner fenders, you don't want the eyes drawn to these areas.
On a Nascar body, spray the inside Gloss Gull Grey, when cured, prep, prime, and spray exterior.