Matt and Neil have been spot on as to how these panels were done. I will refer everyone to an article appearing in the April 1969 BCA Bugle written by Jim Flaherty Jr. BCA 529. Jim was researching this very question. I will paraphrase his findings. The engine turned pieces were manufactured by Croname Inc of Niles, Illinois. Mr. Frank Jassen of Croname described the process. It started with a sheet of cold rolled steel. A group of swirling emery brushes descended upon the sheet giving the swirling and overlapping pattern. After cleaning the sheet was cut and stamped out on a die. A coat of lacquer was then applied. Plain and simple. No decals used here as was done on the instrument panel wood grains.
Matt's pictures illustrate well how the finished engine turned product was then mated to the corresponding heavier metal piece.
With regard to the different colors of lacquer supposedly used depending on the interior color, the 28-41 Part Books show only one part number for the 50-70 series and only one for the 40-60-90 Series depending on the year. Separate part number for new smaller body 40 series for 1941 as well.
Time and ultraviolet light no doubt had differing effects on different panels
Regarding panels that have rusted, I dipped a totally rust obscured radio surround in Evaporust. The pattern jumped right out. Stripped any remaining lacquer, then sprayed clear. Not bad as Thomas B. pictures illustrate.
This posting should put to rest any doubt as to the process used on the engine turned panels for 1940, 41,and 42.