Dan,

You wrote:

"Polishing removes this wonderful layer of Aluminum oxide--right? One would do best to leave an un-coated trailer un-polished--right? The main drawback would be a dull appearance--right?"

RIGHT-ON!! - RIGHT-ON - Hurray - YOU COMPLETELY - Read it!!!

Yes, Oxidation IS better than Filiform Corrosion! Plus it is VERY easy to get off. Leaving you a Bright, Shiny Airstream. So you can shave & your wife can put her make-up on & you don't have to drive a nail into the exterior & hang a mirror on it!!

Seriously though, on Bad Oxided Surfaces, Example: '49 Airstream - never been polished.
Step 1: Wash it.
Step 2: Etch it - to remove ALL oxidation and kill any corrosion, as well - it will thoroughly clean the, "core" out. This process eliminates many man hours of Polishing in order to remove the oxides - PLUS it saves your surface - by leaving the material on. Heavy cutting to remove oxides, removes aluminum.
Step 3: Is what we call, "Coloring". That's where we put the, "Mirror" Finish on your Airstream. We use Supershine to do this with. It has UV Protectants, PLUS it's the ONLY Water Based Polish - MADE!
Step 4: Polymer Sealer- (PURE) This step it performed by wiping on with a Boeing Cert'd, "Scratchless Wipe", which removes, Polishing residues. After Polymer begins to, "Haze" - wipe off or Buff off with one of our Aircraft Drum Polishers. This leaves you, also a UV Protectants Plus a Pliable Sealer to STOP matters from our environment from sticking to the surface.
NOTE: We have been using this 4-Step Procedure on Aircraft for 10+ years. We have HUNDREDS of Happy customers! 18+ years of Polishing in the Aviation Industry with 489 Aircraft Polished - by OUR Polishing Group. Take a look-see at some of them on our web site - airmarktools.net
Step 5: Instruction & training On-Site to our customers - showing you how to maintain your Polished Look to SEVERAL years to come.

Whether you choose to, "Strip & Polish" or Not - Maintain your Airstream with a good Sealer.

Regards,
Jim Weston