The AISC Specification for Structural Steel Buildings provides that, in general, steelwork to be concealed within the building need not be painted and that steel encased in concrete should not be painted. Inspection of old buildings has revealed that the steel withstands corrosion virtually the same whether painted or not.
Paint is expensive to apply, creates environmental concerns in the shop and can create a slip hazard for erectors. Environmental requirements vary by region. Permitting flexibility in coating selection may lead to savings. When paint is required, a shop coat is often applied as a primer for subsequent field coats. It is intended to protect the steel for only a short period of exposure.
Many fabricators have invested in the equipment and skills necessary to apply sophisticated coatings when required. Compared with single-coat, surface-tolerant primers used in normal applications, these multiple-coat or special systems are sensitive to cleaning and applicator skill. While these sophisticated coating systems are expensive, they can be useful when life cycle costs are considered in very long term exposures or aggressive environments.
Steel which is to be painted must be thoroughly cleaned of all loose mill scale, loose rust, dirt, and other foreign matter. Cleaning can be done by hand tool, power tool and a variety of levels of abrasive blasting. Abrasive blasting in most fabrication shops is done with centrifugal wheel blast units. The various surface preparations are described in specifications by the Society for Protective Coatings. Unless the fabricator is otherwise directed, cleaning of structural steel is ordinarily done with a wire brush. Sophisticated paint systems require superior cleaning, usually abrasive blast cleaning and appropriate quality systems.
Knowledge of the coating systems, equipment maintenance, surface preparation and quality control are all essential.
Treatment of structural steel that will be exposed to close public view varies somewhat from that for steel in unexposed situations. Since surface preparation is the most important factor affecting performance of paint on structural steel surfaces, it is common for blast cleaning to be specified for removing all mill scale on steel that is to be exposed. Mill scale that forms on structural steel after hot rolling protects the steel from corrosion, but only as long as this scale is intact and adheres firmly to the steel. Intact mill scale, however, is seldom encountered on fabricated steel because of weathering during storage and shipment and because of loosening caused by fabricating operations. Undercutting of mill scale, which can lead to paint failure, is attributable to the broken or cracked condition of mill scale at the time of painting. When structural steel is exposed to view, even small amounts of mill scale lifting and resulting rust staining will likely detract from the appearance of a building.
On industrial buildings, a little rust staining might not be objectionable. But where appearance is of paramount importance, descaling by blast cleaning is the preferred way of preparing the surface of architecturally exposed steel for painting.
Steels are available which can be exposed to the weather and can be left unpainted, such as A588 steel. This weathering steel forms a tight oxide coating that will retard further atmospheric corrosion under common outdoor exposures. Many bridge applications are suited to this type of steel. Where the steel would be subjected to salts around expansion devices, owners often choose to paint that area. The steel that is to be left unpainted is generally treated in one of two ways, depending on the application.
For structures where appearance is not important and minimal maintenance is the prime consideration, the steel may be erected with no surface preparation at all. While it retains mill scale, the steel will not have a uniform color. but when the scale loses its adherence and flakes off, the exposed metal will form the tightly adherent oxide coating characteristic of this type of steel, and eventually, a uniform color will result.
Where uniform color of bare, unpainted steel is important, the steel must be freed of scale by blast cleaning. In such applications, extra precautions must be exercised to protect the blasted surfaces from scratches and staining.
Steel may also be prepared by grinding or blasting to avoid problems with welding through heavy scale or to achieve greater nominal loads or allowable loads in slip-critical bolted joints.
(Steel Structures Painting Manual, vol. I, Good Painting Practice, vol.II, Systems and Specifications, Society for Protective Coatings, Forty 24th St., Pittsburgh, PA 15222.)