it is an Airstream thing
It was a factory application. I do not know when it started. My 95 Sovereign
had it. I am putting it on my 56 Cloud...previous owner had it done about 15 years ago. While I think the original idea was to reflect heat (what I want it for) some folks use thick rubberized coatings...I guess in the hope to prevent leaks. I am using Ceramaflex white. use search function on ceramaflex and you will get some discussions.
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."