Originally Posted by
dixi188
It looks to me that the bare metal has been Alodine treated ready for painting.
(Alodine or Alochrome is an anti corrosion treatment for aluminium alloys.)
The American MD-11s, like most of their fleet at the time, were not painted; they retained a natural aluminium finish. The exceptions were their Airbus fleet, initially the A300, which had a surface finishing process which did not suit this, and thus were painted in a grey-lookalike colour. American had standardised on this unpainted (apart from decals and cheatlines etc) finish ever since metal aircraft came along in the 1930s, until finally changing over to paint about 7 years ago, possibly not quite fully achieved yet.
Having said that, the finish was not easily achieved, and at the start it required quite some treatment to achieve, which American normally did themselves at their central maintenance base in Tulsa OK where aircraft were delivered. These aircraft however were unfinished, and were sent to Marshalls in Cambridge for whatever remaining works could not be carried out at McDonnell Douglas in Long Beach. Don't know whether the protective coating was stripped and the metal finished by Marshalls, or if they were then taken back to the US for the final preparation.
The surface treatment took more work to prepare and to maintain (polishing), but gave a weight saving. Their accountants must have calculated it was worth it one way. Many other airlines of course thought the opposite, though there was a spate of "natural" finishes by other carriers in the US around the 1990s. It's particularly difficult to achieve on a secondhand aircraft that has been previously painted, and American abandoning it coincided with the merger with US Air which brought large numbers of painted aircraft into the fleet. Those from London may recall that on the Underground, all new trains were unpainted aluminium from the 1960s to about 2000, when a paint livery was applied to them all. This was principally driven by the paint graffiti vandals, it being extremely expensive and time-consuming to get back to a decent finish afterwards. The very first unpainted aluminium Underground train carriage was sent by the manufacturers to a trade exhibition, for which it was got to an attractive mirror finish which was still somewhat apparent compared to the rest of the fleet 15 years later. Apparently this had involved a huge effort, and wiped out the stores stock of fine-grained sandpapers !