To add to PJ's exposition, here are some drawings that I have found helpful. I recently bought a book about building the Mustang with many production photos and these drawings don't seem to argue with the photos.
The Jennings Helig photos show where the silver lacquer was applied but there is much more detail in the second two about primer and filler application.
The access doors in the leading edge ahead of the landing gear were not painted in the photos in the book, where Jennings indicates they were and the second drawings indicate they were not. The primer and filler were applied during wing construction, not after final assembly..
