tell him to put train horns on the mail truck
That wear ring is normal. Just use a brillo pad to clean it up. You dont want any paint anywhere the stainless plates touch. You take the risk of changing the tone for the bad, or worse they will only squeal.
Train horns on a mail truck would be the poop
AC COIL cleaner is a good product have used it personally but i do rec. that you dont leave that cleaner on long or its like an acid product and will stain your horn so basically you will have spots on your horns and you dont want that.
i use that same aluminum brightener…it works badasssssssss