When painting plastic and rubber parts, good shops add a "flexing agent" to the paint. This gives the paint a "rubbery" quality, so it can conform and deform to the movement of the surface it is adhering to. Metal is more rigid, so those qualities are not necessary for paint in those areas.

Even if you prepare the surface properly, you will continue to experience "spiderwebbing" or "flaking" unless the paint is properly treated. This is not as big a problem with the harder plastics, but the soft plastics like rubber moldings have a larger expansion rate, and are more prone to the problem.

BTW, the one way a surface can be resistant to rock and debris chips is by powder coating. That process however is electrostatic (requires a conductive surface), and thermal. The temperature of the baking process would melt the plastic pieces.

The other is to use Imron (a Dupont product). It's widely known as aircraft paint. Extremely durable, but difficult to apply, and even more difficult to remove if you screw up...I've had metal engine tin pieces that were subject to a lot of dropped tools, etc. painted with Imron, and they held up extremely well.

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2009 ML350 (106K) - Family vehicle
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2016 Mustang (32K) - Daughter's car