Over its 100 plus year lifetime, the paints than adorn cars have evolved from natural products to fully synthetic ones and the method of applying them has changed from laborious hand brushing to fully automated methods.  Here's the story:

In the first part of the 20th century, automobiles were painted completely by hand.  The paints were usually linseed oil and natural pigments and the painting process was a lengthy affair. Back then, painting a car involved several days of drying time!  This part of the production process was a serious bottleneck in the automotive manufacturing process.

DuPont developed a greatly improved paint system in the early 1920s with its lacquer paints.  These paints were based on nitrocellulose and were applied via spray guns. This speeded up vehicle assembly time a great deal.  They weren't perfect paints. Lacquers also have poor resistance to certain petroleum-based solvents. Repeated exposures to gasoline spills, for example, could damage lacquer finishes. 

In the 1930s, enamel paints came about. Enamels were sprayed on vehicles and then were baked in ovens. This speeded up the manufacturing process a lot and produced a rock hard finish which was resistant to solvents. However, enamel paints had a down side.  They oxidized in direct sunlight which caused the colors to begin to show fading and/or dulling quickly, sometimes in just a few months. 

In the late 1970s, a new type of finish, called "basecoat/clearcoat," was developed. Basically this type of paint consisted of a pigmented enamel basecoat followed by an ultra-hard clear enamel top. It allowed a great deal more flexibility in the composition of the base coat (more colors and metallic) because the top clear coat functioned as a protectant by incorporating UV absorbers. And they were beautiful to look at. 

While the basecoat/clearcoat paint system is far superior to conventional one-coat enamel paints in many respects, it has a few disadvantages. The clearcoat has a greater tendency to show marring when rubbed by foreign materials.  Finally, excessive polishing a car with clearcoat on it can remove a good part of the finish along with its built-in UV protectants.Â