Powder-coating would be more durable than spray-paint, but if you want to spray-paint anyway:
-- rough-up the finish with 600 grit sandpaper
-- wipe with denatured alchohol
-- spray 1st coat, don't try to put on too much, the paint will run (if that happens, use the 600-grit paper to level the runs with the surrounding paint,
-- spray 2nd coat
-- spray 3rd coat,
-- then 2 or 3 clear spray, again, light coats, so the clear doesn't run.
Spray paint will still chip and scratch, especially around the side panel latches.
I know, but I've sprayed several cases, and they still look good, except for the chips and scratches on the latches - which show when I take a photo of the internals. Just take your time and let each coat dry before you apply the next.