I also had some sort of stain that refused to "clean" off. Showed it to a PPF shop, and he came out with a portable polisher and took care of it.I had a bird poop stain on 3M that was installed 6 months ago. I used Griots Complete Polish with their microfiber polishing pad. No issues and stain is gone.
Excuse me in advance, but the concept of polishing paint protection film, somehow, to me, seems really weird.@cerbomark ,
My PPF is ceramic coated, I took @DFB5.0 advice on cleaning, and no polishing is required. Hope this helps.