Does the Air (and the MBP I presume) have a painted aluminum finish, or is it just the actual raw aluminum?
All Apple products that look like aluminum are actual aluminum. Faux finishes make Steve Jobs angry.
The aluminum on MacBooks, iMacs, and iPhones is anodized. This is a chemical process that creates a layer of oxidation on the surface (the characteristic "silky" feel of new Apple hardware). It's not just cosmetic; it also prevents future oxidation, which may stain and damage the material.
For this reason, it's definitely not recommended to sandpaper or otherwise remove the surface on your MacBook Air. You'll be exposing untreated aluminum to oxidation.
I'm with DaveOZ. Who cares about scratches - especially ones on the bottom. If you want to keep it pristine for great resale value, either guard it with your life or buy a shell case. Me, I'd never put a beautiful piece of hardware in one of those plastic monstrosities. To each his own, though.