Zinc Chromate on Outer Skins

Working on a B-29 restoration project, I was told they did not originally use ZC since the AC was not expected to be in service that long due to combat loses. Was this just a story, or common practice by some AC manufacturers?

Like any other paint jobs, a good prep and primer are necessary for good results.

Yes it was. I don't know on what, when, or where, but I have seen chipped paint with ZC underneath

I have found photos that show the wing folds and wheel wells of an F-6F painted with ZC.  Try Googling F6 Hellcat images.

Take a look at this thread I posted:

cs.finescale.com/.../161858.aspx

Zinc chromate primers were used as primers on aluminum, but then overcoated with color paint. In many cases they seem to have been used as a final finish on interior parts that were protected from the environment, but even for those interior areas, they were often painted with a finish color.  Of course, for those planes left in bare aluminum there was no primer, but if the exterior was painted, chances are good that it was primed in ZC.