I don't kniw what GF product you are using, but every can of their 'Milk paint' I ever opened was darn near DQ 'Blizzard' consistency. I can't imagine spraying it. However, a foam brush gives me a very smooth finish, as the stuff self-levels quite well. But....I always seem to need 3 coats to get non-translucent coverage.
Cabinet doors really benefit from sanding after assembly, a drum sander (or wide belt sander for higher volume) might be worth adding to the list.
If you are doing thus commercially, go wide on the jointer. Hobbyists can afford the extra time to use planer jugs & such, to get by with a narrow jointer, or none at all.
Mark the center point for the domino on the ends of the box, the tap a small finish nail int the mark. Snip it off about 1/8" above the surface, and it becomes the 'dowel center' to mark the overhanging top & bottom pieces.
Drill and appropriate pilot hole to make the cut off nail easy to extract.
Here's an example of dominoes in the middle of a flat surface.
Using scrap blocks for an offset fence make this pretty intuitive once you learn to think upside down and backwards . Seriously, it all boils down to using a common reference surface for the mating mortises (mortisi?). You can also use one tight mortise and other loose-fit mortises to make lining things up easier. there will not be a terrible amount of stress on this joint, right?