A bit about aluminum sheet. 2024 is a stronger alloy with a lot of copper, which makes it heat treatable and why it is used for aircraft skin, the most common. It is, however, not at all good for polishing unless it is clad. It is also not weldable. I would be surprised if it is used on the Airstream exterior skins. The alloys we polish in Aerospace are typically 5000 series and 7000 series. They have zinc which is part of what makes them polish well. 5083 polishes very well. in my former job we polished it a lot, and made body panels for Aston Martin, Morgan, and the Ford GT from it. Some aluminum alloys do NOT have to be alclad to be polishable. However, certain alloys that do not polish well in the base material, need a layer of pure aluminum (alclad) so that it polishes well and it also helps with corrosion prevention (eg 2024). 5000 series does pretty well avoiding corrosion without needing to be clad. 5052 is another common alloy. i don't think it polishes as well as 5083. 3000 series is almost pure aluminum. it polishes well but is not very strong, mostly in tensile. Old Shelby Cobras would be made from a 3000 series. Easy to form but not strong. Flexural strength or stiffness, crumple resistance, is a concern for body panels. if you need to cold form the metal into contoured shapes, 3000 series is the most ductile but again, not as strong as you might want.