If we want to successfully perform any surface tratment repairing process, we must first understand how the manufacturer is applying the coats and prepairing the piece/vehicle in order to achieve its final result.
Basically, the body is washed, degreased, electrodipped in zinc phosphate, oven baked, sanded, sealed, dried, cleaned (with chemicals), primered, baked, painted, baked, clear coated, baked and then inspected for faults (which are repaired). All high volume manufacturers have fully automated lines that take between 14-18 hours to finish a single body.
Even if the paint process may be different among manufacturer & sectors, the main process looks something like this:
The result is layers of carefully laid out coats, all of which put together is only slightly thicker than the average human hair.
Cathodic e-coating gives the car body a primer coat. It protects the car from corrosion , especially the cavities of the body that cannot be reached easily by spray application.
Primer: evens out any irregularities of the underlying layer. It can be sanded easily and provides a smooth surface.
Basecoat: gives the main color to the piece, therefore it plays a significant role in establishing the final visual impression.
Clearcoat: seals everything else and is applied last. It protects the car from external elements like snow, UV radiation, dirt, etc…
Why is this important? Because it will help you imitate the factory finishing in your workshop and achieve a better result on your repairs. Next time you perform a paint job, check the background on that piece! Knowledge is everything.
Image source: Automotive Paints and Coatings