Steve…are you referring to the concrete and metal prep cleaner that Paul was talking about in his above response?
Looks like the effect I've seen on my signs. The stuff I used is this. There are other metal etches that do the same thing, used a lot in autobody shops for removing surface rust. Been using this stuff for years on cars, but never thought to use it on signs until recently. If you leave it on too long it'll remove all the rust leaving clean metal (with a grey zinc phosphate coating) where there is no paint at all.