If the stripes were put on in a factory as part of the original paint job, masking would have happened, but probably involving sheets of plywood as a stencil rather than tape. They tended to have paint spraying equipment in factories. In the field, a totally different set of circumstances. A straight edge might have been involved, application could have been brush or broom. It was wartime and the job didn't get held over until perfection could be achieved. What was to hand on site got used.
In my time I've seen Post Office vans repainted using a roller on the flat panels. And if the painter is talented, it is surprising what can be done with a brush. When fitting a switchboard at a local transport company (BVT) about 1964, I witnessed their painter putting their logo on a new trailer. It was a 6 foot image of the Tower, all done by hand and eye, took him under an hour each side.
Painting stripes reasonably straight and of uniform width would be no problem given a brush with enough bristles and a long enough handle. And a sergeant looking over you.
