There was varnish first like on wood furniture, India Enamel (based on India ink, or lamp black, not the country) Japanese (nitrocellulose, or gun cotton used in smokeless powder) lacquer came along in the 20's. It was all applied with a brush and dipping until spraying technology caught on with the lacquer, late 1920's.
It was used pretty much industry wide until alkyd enamel came along in the 30's. It was used until after the war when acrylic lacquer came along. It dried very fast and helped Detroit pump out a lot of cars to face the high demand. It wasn't until the late '50s that acrylic enamel became common. If you see a pink and white '55-'57 Chevy,it was originally much redder. The lacquer they used lacked the UV protection to prevent the "Pepto'Pink" fade. Fender guitars did the same thing, you can tell if its a real '50's survivor by having the faded lacquer. If its enamel and pink it was repainted by a fag.
A short history there, because there were so many developments in primers, undercoats, reducers, activators... You would almost need a book to put it all together.
