What is a tailgate band made out of?

[Chevy3+3]

I have a 78' Chevy wide tailgate band that I want to clean up. There is a couple of little dings in it and in the dings appears to be rust colored so I assumed the panel was a form of stainless steel.

I sprayed some cleaner made specifically for stainless steel and if turned the surface a little chalky/milky. It isnt bad but no matter what I try and polish it with I get the same result.

What is my tailgate band made of? I have heard there is an anadized coating on them also?