They may sit for weeks in a field, subject to the elements. Preprimed steel isn't always handled so gingerly, either. Or it gets scratched on a forklift. Scrape off the primer, of course, and the exposed steel is at risk for rust. It's not just the rough treatment that results in unprotected steel.
Welds can rust, as can nuts and bolts.
They need to be primed, too. The specifications for steel installers in a typical contract require them to take care of the rusty spots, welds, and nuts and bolts. Apply paint to exposed areas using same material as used for shop painting. Contractors can point to these words and refuse to accept new steel as ready for topcoating, but they have to register their objection before they start the job, said Daniels. Daniels said he argues that the painter should only touch up whatever he's damaged himself.