What Is A Red Iron Purlin?
This is a purlin that has been painted with a thin red oxide coating. The coating is intended to prevent rust during shipment. It's not a finish coat of paint and the substrate beneath the paint is a bare steel and not a galvanized finish. The red primer prepares the metal for a top coat of paint. Red purlins should be painted prior to installation. If a secondary paint finish is not applied the purlins will rust.
The main advantage of a primed red purlin is that it will save you 25% to 35% versus galvanized purlins.