Thanks for the encouragement guys!
Steve (SCMc) - interesting point!
I am not a metalurgist (but somewhere in my deep dark past there was an engineering degree somewhere) - The skin was actually duraluminum, which can be susceptible to corrosion with a certain make up of alloys (too much copper I think), so it needs to be coated in an aluminum solution or bonded to aluminum to minimise corrosion. The remainder of the interior structure was likely of welded tube structure. Depending on the grade of steel used (likely to be a harder grade of the 7075 ilk, which is also suceptible to corrosion), there would be potential for the steel to corrode if not coated, as well as dis-simillar metal corrosion possible throughout the interior where the differet metals joined. This was not a problem for metal fittings in wooden aircraft but for an all metal aircraft in an era where dis-similar metal affects of new alloys were not well known, I figure a coating was likely present, which may or may not have had a clear (clad) or painted appearance.
Not sure if I am right here (in fact I am likely to be wrong, it has been a while)...
BC