from 1962-1972 the bare aluminum on the outside of Air Force One (the VC-137C) was so highly polished that they used baking flour as an abrasive to polish it.

Anything more abrasive than flour would scratch the pristine shine they had. 

 ‘And they said that “Gold Medal Enriched” was the best for polishing. 

 ‘So, that’s how scuff marks on the shiny bare aluminum on the outer skin of 27000 and 26000 were removed back when I was a pilot in that wing.’ 

https://theaviationgeekclub.com/89th-airlift-wing-pilot-tells-why-gold-medal-enriched-flour-was-used-to-remove-scuff-marks-of-the-vc-137c-air-force-one/