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I use to restore antique slot machines made from aluminum. After a high polish I would wipe the 'film' with acetone, outdoors. It leaves no residue of oil, as I would then tape off the polished areas to paint and bake on the enamel colors.
The Black Residue is actually... Aluminum and Polishing Compound. Aluminum's streak test is 'white' in powdered form. I would probably say... silver, but have not tried it on aluminum.
Many elements / minerals when scratched against a white rough back of a tile may have a different color than you see. The 'streak test' used by geologists and mineralogists. Aluminum's Mohs hardness is 1.5 when not alloyed, so is soft. Your finger nails have a hardness of Mohs 2 to 2.5 and glass around 5.5 +/- meaning your finger nail can scratch pure aluminum.
Some Aluminum Alloys polish into a nice Silver, as the 1930's Mills Slot Machine castings. If you look at the stamped front and rear sections of Airstreams, you will see the aluminum appears to be different. Someone might be able to explain that better than myself, but take a look at your later Airstreams curved sections. The older models may match the rest of the aluminum 'color'... since the sections are curved and riveted and not stamped into shape to save labor costs.
There is no doubt unlimited Aluminum alloy products for various applications. No doubt Airstream has changed alloys as the older models are silvery and the newer models with the 'sealer of some sort' have a greyish appearance.
When the aluminum gets 'hot' when polishing... it will polish to a brilliant silver on older models. I had a 'red brick' for getting through the layer of oxidized aluminum, wiped off with acetone... then the 'white brick' for the final polish. It has been thirty years since I restored slot machines... but the supply shops know exactly what is best for your needs. Check with them... first.
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Human Bean
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