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This might help to show the wing upper surface Alberto.
Somewhere I have the official specs for the finishing of the wing, and from memory, the 40% is correct. This takes it back to the line of the main spar, where the area from leading edge to the spar line was puttied, polished and sealed.
However, all of the remaining panel joints, except access hatches, were filled, and the entire wing surface finished in a coating of 'aluminium' paint, or dope, made from clear lacquer and aluminium powder. The flaps and ailerons, and the wing tip fairings, were bare metal, as can be seen in the photo I posted.