HighPlains,
The outer section of wing, from the fold joint out to where the new plastic tips went, was fabric covered from day one. The LE was aluminum back to the spar. Ailerons were also aluminum structures covered in fabric. and....
Considering the flight date of 4th April 1946, it is strange to reflect that the F4U-5 was the first Corsair to have metal skin replacing the fabric that had covered the outer wing panels since the XF4U-1 flew on 29th May 1940.
Profile Publications,
The Chance Vought F4U-4 to F4U-7 Corsair (no 150)
So all through WWII, the Corsairs had fabric wings.
The usual reason for having a fabric covered surface was weight and ease of construction. Also, during wartime, fabric covering utilizes non-strategic materials and cheap, available, already trained labor that comes from outside the high-tech labor force.