Right, so most consumer paint guns have trouble with oil based paints, so thinning it to go in the gun is a bad idea, because you destroy the point of oil based.
Oil based and water based both work well, but here's what I prefer, and it works very nicely.
1). use rollers to manually primer the box.
1a). I always use two passes of primer with rollers
2). I use water base part, Semi-gloss in my spray gun for the color I want.
2a). Second pass on the gun is always critical to get a good covering.
This process works like a beast.