I toured a GM auto plant two years ago and had the chance to see their production painting system. The painting robots use a device that atomizes the paint to help ensure a pretty even coat. The problem with this system is that for primer/sealer and both base and clear coat the thin atomized paint will "pile" on the uneven primer/sealer creating what we call orange peel. They no longer sand smooth the primer/sealer to create a flat painting surface - takes too much hand labor which equals $$$$. You would think that as the cars move up in price range, they would paint them to higher specification, but they receive the same painting system albeit sometimes using a "better" paint. Regardless, mass production painting using this system will result in a less then perfect finish.
Other than some exotic car manufactures, body panels are not sanded smooth following the primer/sealer and base coat stages. Those that are show a remarkably better finish, but I have seen Ferraris with slight amounts of orange peel on some panels.
If you have even seen a Concours winning car or other custom painted car, you will drool at the perfect paint. I was at the Barrett Jackson auction a few years ago and there were a few cars that had flawless, two inch deep looking paint on them. They were glass smooth and reflected like a polished mirror.
One of the cars had a picture book of the paint process. They used a metal etching sealer/primer, then four layers of primer/filler blocked and wet sanded after each coat. Six layers of base, again blocked and wet sanded after each coat. It was then followed with two pearl layers blocked and wet sanding the first layer. It was finished with three layers of clear blocked and wet sanded. The final coat of clear was allowed to naturally cure and then it was rotary polished. It was stunning.
-Sparty