Certainly by World War I, and I believe earlier, stencils were used, then painters filled in the "stencil gaps" by hand.
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It was also possible to use lengths of wire in place of the stencil bars; this was often done when the stencils were made of sheet metal. The relatively thin round wire allowed the paint to flow around and under them, so there were no gaps to touch up, although on a newly stenciled car you could see the marks the wires left in the paint surface.
For those wondering, before the days when spray painting was common, stenciling was commonly done with "stencil brushes" round short bristled things that looked like stiff shaving brushes, used with a stippling motion. The paint used was commonly called "stencil paste" and was heavier bodied and less runny than standard paint. It was slow drying and tended to self level as it dried.
