Having spent almost 30 years around industrial/automotive painting, aerospace painting, and painting using robots, here's a few things that may clear some things up:
1. Plastic parts are generally painted at the same factory where they are molded, with the part going from mold to painted anywhere from a few hours to a few days. These parts are cleaned via 3-stage or 5-stage industrial cleaning systems, destat'd (removal of static charge), and prepped before painting and handled with gloved hands. There are NO oils, silicones, etc... on these parts when they receive their OEM finish. They are painted in quasi-controlled environments, maintaining as constant of temp and humidity as they can.
2. OEM paint is not necessarily the same as the paint you can buy to match the OEM color. Most factories have agreements with various mfr's for certain chemistries, characteristics, additives, etc... and they consider them proprietary. Not in all cases, though. In any case, many plastic parts are not primed at all, the solvent used in the paint literally softens the surface plastic and chemically bonds the paint to the plastic, kind of like airplane glue (plastic model glue) softens mating plastic surfaces and bonds them together. In fact, the solvent in airplane glue is Xylene, which is used in many OEM paints. It basically microscopically bonds the paint to the plastic by "melting" the very surface layer of plastic, allowing the paint and plastic to "mix" and cure together as the solvent evaporates out of the system. There are more and more plastics being pre-coated with a very thin coat of an adhesion promoter (or primer, if you will) that also has some additives in it that will allow the part to be sprayed electrostatically.
3. These parts are all force cured using ovens, IR lighting, etc. Typically, an ambient cured coating will never achieve the properties that a force cured coating will, no matter what any "expert" or sales/marketing ahole tells you.
4. There is no difference in surface finish between a robotically sprayed and a human sprayed part. A human will often impart variability into the spray and get sags, runs, light spray, heavy spray, etc..... but when a good human spray job is compared to a good robot spray job, you wouldn't be able to tell the difference. Finish quality, all other things being equal, is dependent on the paint chemistry, viscosity, spray gun (atomizer), build rate, and environmental conditions. What a robot provides is repeatability and uniform control of the spray gun motion, standoff, speed, etc. It doesn't get tired, have hangovers, have girlfriend issues......
5. most plastic parts are now sprayed with electrostatic rotary bell atomizers. These spray "guns" spin a very finely balanced bell cup up to about 50,000 RPM's and mechanically atomize the paints into extremely fine particles.......more so than the classic style paint gun, and much more uniformly...... and throw out a plume of paint that is charged via mechanisms in the spray gun. The parts are grounded (they are made conductive via the additive in the adhesion promoter) and the result is the charged paint is magnetically attracted to the part, even wraps itself around curves, edges, etc. The very finely atomized paint lays down smoother and flows together better, which is what produces the superior surface finish.
6. When you repaint parts, there is virtually no way to reproduce the cleaning process, decontamination, prep, etc... as good as the factory does, so you are likely never going to get as good of a paint job as the OEM. That said, there are places out there that produce extremely good paint jobs, but this is one of those things where you get what you pay for. There is a reason why good quality paint jobs are very expensive: they don't shortcut steps, they have the proper facilities and equipment, they are more experienced at matching paints & chemistries, along with matching paint chemistry to primer chemistry, etc.
