I've read several times that in WWll the standard order for painting in the field was, "use what you have." Jgeratic has it right, lot's of A/C left the factory with the appropriate camo applied, but if the situation was desperate in theatre, they might just get test flown and head out bare, painting to be applied however possible on arrival. Small repairs or minute unit markings were often applied with brush, in some cases the unit might have a compressor and portable tank, then paint was sprayed either free hand or using the pre made masks if available.
In the field I'm fairly sure the camo work was done freehand for the U.S. A/C, I believe the "splinter" camo on German A/C might well be done with pre made masks.
The objection noted about brushing, it applied much more paint than did spraying and that was a minor weight penalty, plus field contamination and minimal cleaning equipment on hand left a surface that didn't retain paint all that well, often large sheets of it would tear away at high speeds. The spray guns were indeed commercial types as mentioned, some were full size and some were smaller, called "detail guns," but still much larger than an airbrush.
Those were tough conditions to work in, dirty, oily, combat repairs galore, it's a wonder they were able to do as well as they did. Many years ago I saw a B-26 in desert storage, left as it was on return from the war, it just never got broken up for scrap yet. The condition was horrible,many huge dents on the fuselage sides adjacent to the props, I suppose from ice leaving the blades and slamming into the sides. Massive areas of paint flaking away all over, and the entire airframe looked as filthy as any truck or jeep that I've ever seen after combat field service.
Those were tough conditions for crews to work in, it took genuine dedication to get the job done and the airplanes really took a daily beating, it was combat after all.