Our '53 Buick hardtop is now in primer and we just finished hand "blocking it down" wet, using 240 grit wet or dry sand paper over a 12" sanding block.  There are dents as well as high and low places all over the car.  As we finished sanding each panel, we used a black "sharpie" marker to circle and note each place to repair.  When it is time to repair the damaged areas, the notes will be sanded off.  If left on, they will come back through the paint.  As we worked, bringing the old Buick's body back to Harley Earl's standards, we talked about how the automotive refinishers would have done this in the early days of car building, and we thought it might be interesting to feature
The History of Automotive Paint.
        In the early days of car building, the newly-formed car companies used the carriage makers' painting process of hand-painting each vehicle using brushes.  It was a slow, laborious process that could take several weeks to finish one car, depending on the weather conditions.  The unmounted car bodies were brought to the painter in primer.  After the bodies dried, they received more primer coats, each one brushed on at 90 degree angles to the previous coat to hide brush strokes.  The body was moved to a drying booth, then it was given a yellow top coat which was sanded off to show the high and low spots on the body.  A couple of base coats were then applied and rubbed down with compound, then the color coating began--a process in which the paint pigments were added to clear varnish, giving color to the painted surface.  The color was built up to as many as six coats, rubbing down each one to level the surface.  The final coat was applied "heavy", allowed to dry, then rubbed down, pin striped, and finished off with a couple of coats of clear varnish.  In spite of all this work, the finish usually lasted only two years or so before needing to be refinished!
        In the days before the assembly line, when cars were hand-built, waiting for the paint to dry was not too much of a problem.  As production time was reduced and the cars were rolling out faster, however, a new, quicker paint process was needed.  Henry Ford (and others) had begun using "baked-on" enamel as early as 1914 on his Model T's.  The problem was that colors couldn't hold up to the 400-500 degree baking temperatures, nor could the wood in the bodies.  The only color that could stand the heat was black!  A system was developed to lower the heat to around 160 degrees, which was not harmful to the wood bodies.
        In 1923, General Motors along with DuPont, developed a spray painting system using "nitrocellulose lacquer".  DuPont had actually been producing lacquer since 1905, but had not applied it for automotive use.  This new paint, called "Duco", could not only be sprayed, but had an array of colors.  It had to be rubbed, but did not need to be baked.  To apply Duco, the body was metal-prepped, primed and sanded, then the color coats were applied, wet-sanded and buffed.  These lacquers produced a beautiful, deep finish that was much quicker to apply, and were used until the 1950s.  Although beautiful, lacquers had poor resistance to gasoline spills and other chemicals, and had to be rubbed out periodically to remove oxidation caused by the sun and restore the color and gloss.        
        By the early 1930s, "Alkyd" enamels were developed.  These paints were more durable than lacquers, but required baking to make the paint flow and produce a smooth finish.  Like lacquer, they oxidized and caused the colors to fade, but overall, they produced a good finish.  By the mid-1950s, GM developed a new acrylic lacquer system, which produced a smooth, durable finish that held up bettern than the old nitrocellulose system.  It didn't have the depth of nitrocellulose, but required less maintenance.  By the late 1960s, acrylic enamels became available.  When used with hardeners, they could be color-sanded and buffed to produce a high-gloss finish with similar depth to lacquer, and stood up well to weather abuse.
        In the 1980s, under governmental mandates to lower the "VOCs" (volatile organic components), paints were developed to comply, introducing the "base coat/clear coat" system.  These finishes feature greater durability and very high gloss, but less depth than the old lacquer finishes.
        As we paint our car today, it is still a labor-intensive job.  Time should be spent at the beginning of the restoration, to determine what look we are going for:  An OEM finish that is deep and glossy, or a base coat/clear coat finish that is less deep but even more glossy.  Whatever finish you choose, it affects the character of the car.  We always do a sketch of the car first, and color in our paint scheme, to decide which paint system (lacquer, enamel, etc.) will be right for the car.  Once that has been decided, we choose the paint manufacturer (we use PPG) and stay with that company's products throughout the project, so that each product will be compatible with the next.
        Automotive painting can be a lot of fun and the result can be something that you're really proud of.  It's worth the work and research, so that you will achieve that finish you envisioned when you started your project!  Enjoy your cars...keep 'em driving!
REFERENCES:  "A Century of Automotive Style/100 Years of American Car Design" by Michael Lamm & Dave Holls