Paint colour for backing plates ?

Do you know what they were painted with ?
I know it sounds hokey but the vinegar thing really does work.
I was sceptical when I first heard of it but figured what the hell.... 2$ / gal .
I soaked my backing plates and they looked great.
I painted mine anyways because I wasn't concerned about factory look and nobody ever sees them.
I think they were primed & then painted with tremclad or rustoleum. I can't see vinegar cutting the paint off? Did you actually do that? Thanks.