Sand, prime, first coat.
Finer sand, second coat.
Rinse/Repeat till desired finish achieved.
Apply top coat/gloss.
Sand, second coat.
Rinse/Repeat
Key is several light coats not laying it on like painting a wall.
Honestly I would look for a powder coating instead. My last case was a massive Corsair 900D and it was like $100 to have it coated. And an added bonus of powder coating is it is much more durable than off the shelf spray paint.