8.3 Applied Coatings and Paint
Coatings and paint applied to galvanized steel connectors can improve corrosion resistance, but many coatings and paints commonly used for buildings do not adhere well to galvanized surfaces. The Truss Plate Institute (TPI) has evaluated the use of truss plates in corrosive environments. If specified by the registered design professional or building designer, ANSI/TPI 1-2014 design specifications, which are referenced in the I-Codes, recommend that one of two industrial paint systems be applied by brush to embedded plates after delivery of the completed truss to the job site or after truss installation (ANSI/TPI, 2014). Alternatively, using truss plates with a thicker hot-dip galvanized coating is also recommended. The three industrial coating options for increasing corrosion resistance are:
- Epoxy-polyamide primer (SSPC-Paint 22)
- Coal tar epoxy-polyamide black or dark red paint (SSPC-Paint 16)
- Post-plate manufacture hot-dip galvanizing (zinc-based) per ASTM A153/A153M (ASTM, 2016)
The degree of improvement in corrosion resistance that the recommended applied coatings provide is difficult to estimate. Unlike differences in galvanized coating thickness, differences in paint system coating thickness do not proportionally change the corrosion resistance of the connector. Coating lifetimes are significantly affected by salt spray, but exposure conditions can affect coatings and galvanizing differently. Surface preparation and care in application are critical for improving corrosion resistance with coatings. The added cost of these coatings varies with local labor costs.
In general, other types of coatings and paints should not be assumed to significantly improve the corrosion resistance of nominally galvanized connectors and truss plates. For other types of connectors, the alternatives described previously are recommended over any other type of painting. However, for maintenance, zinc-based coatings may offer some benefits over standard coatings because zinc-based coatings can increase protection somewhat in areas where the coating is damaged. Stainless steel metal connectors should not be painted because the coating can prevent the protective oxide film that forms on the surface of stainless steel from developing. This oxide film is how the stainless steel resists corrosion.