Question for the Experienced Body and Paint Guys

Not sure single stage is a good idea...read on.

I agree, jamb everything first, assemble, then paint it all together. Research how to mask so that it does not leave a sharp paint edge..lots of youtube videos out there on that.

All that said, I am NOT a fan of single stage when it comes to mettallics and the reason being is you can get what I call zebra striping from how the metal lays out in the paint film. Depending on if the flakes lay on edge, random, or lay flat, light will reflect differently and you can probably get 10 different shades. Air pressure, distance from the car, angle of the gun will all affect this so consistency is important to minimize this effect and always keep gun perpendicular to surface. This will show up worse under street lights at night.

Another thing that can happen is a condition called flip flop. This is where, even on a vehicle painted assembled, each panel can look as though it was painted separately, if you paint spray the front fender, then jump over to the other side to keep the paint wet, then jump back to the fender area and paint the door next to it, etc... It is best to paint down the entire side of the car (House of Kolor has some great videos where it's founder demonstrates this method of eliminating flip flop). Another fix for flip flop on single stage is to "fog" the final coat so the metal flakes are all laying the same.
The problem with this is that you may get a dull final coat which you will want to buff out...which will expose the stripes...which is why I like to use clear over urethane so that you can fog, clear, then cut and buff the clear and leave the color alone.