When did it start? i'd guess the 70s below is a video of how they did it back in the day.
The painting at the factory involves a huge vat of paint and just dipping the frames, then the detailing was done with a fine gold roller which was set in a sort of jig. Jump to 7 minutes, to see the painting process.
As for applying paint, the saving that you might make by buying it in a tin and applying with a brush must be minimal...i doubt there is a saving when you take into consideration the extra time it takes.
I used to really dread spray painting anything after tales of how the finish was always rubbish, but once you get the hang of it...there is no looking back.