These tables offer rec ommendations for the type of

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application guide showing the most effective use of the paint systems described in Table 1. These tables offer rec- ommendations for the type of system that will be effective, based on the severity of the environment in which it will be used, and also indicate the systems that can be used to topcoat various types of existing paints. Interior Structural Steel Before an appropriate coatings systems for a specific application is determined, it must first be determined whether or not a coating system is actually required at all. Currently, many architects specify all interior steel that is not covered with spray-applied fire protection to be shop primed, even though the steel will not be exposed to view or subjected to corrosive environments. This specification is usually not appropriate and is generally not in the best interest of the owner. An examination of a number of buildings that had been in use for more than 50 years indicated no corrosion of any significance whether or not the steel was painted. Some isolated locations of severe corrosion had been found in these buildings, but only at localized spots where water had been allowed to seep in and remain in con- tact with the steel for long periods of time. Results of this study led the American Institute of Steel Construction to conclude that structural steel hidden between the exterior cladding of a building and the interior finish need not be painted.
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PAGE 37 SYSTEMS Appropriate protection of the steel should be determined by the end-use of the building and the exposure of the steel structure. The building's service requirements may determine that little or no protection of the steel is nec- essary at all. Steel does not rust except when exposed to atmospheres above approximately 70 percent relative humidity. Serious corrosion of steel occurs at normal temperatures only in the presence of both oxygen and water. In dry atmospheres (less than 70 percent relative humidity), non-painted steel can be exposed for extremely long periods of time with no evidence of rusting. If the steel is not painted, a thin transparent film of iron oxide forms on the non-painted steel, actually protecting the steel from further corrosion. Therefore, it is difficult to justify painting all interior steel members as a protective measure for the steel. SYSTEM PRIMER INTERMEDIATE COAT TOPCOAT 1 Inorganic Zinc-Rich Primer Epoxy Aliphatic Polyurethane 2 Waterborne Inorganic Zinc- Rich Primer Acrylic Waterborne Acrylic Waterborne 3 Polyurethane Organic Zinc- Rich Polyurethane Aliphatic Polyurethane 4 Epoxy Organic Zinc-Rich Epoxy Aliphatic Polyurethane 5 Epoxy Epoxy Aliphatic Polyurethane 6 Polyurethane Aluminum Primer*** Polyurethane Aliphatic Polyurethane 7 Epoxy Mastic Epoxy Mastic Aliphatic Polyurethane 8 Oil and Alkyd* Oil and Alkyd* Oil and Alkyd* 9 Acrylic Waterborne** Acrylic Waterborne** Acrylic Waterborne** 10 Polyurethane Micaceous Iron Oxide*** Polyurethane Micaceous Iron Oxide*** Aliphatic Polyurethane Newer Technology Paint Systems 11 Polyurethane Organic Zinc- Rich Polyurethane, High Build 12 Polyurethane Organic Zinc- Rich Polyurethane, water-based
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