Here is a link that Epic started about PBR’s. So I figured since this is ArchViz we as the community can contribute to this. Maybe with some screen shots of each material breakdown or just getting/giving advice about certain mats.
My first question is whats the specularity of normal paint on an iron beam?
In that document, Epic says that 99% of materials don’t need to have their specularity adjusted. The default is 0.5, which should work just fine on paint. And it’s also important to know that the iron beam has nothing to do with the material values. You are only worrying about the outermost surface, which is simply paint.