Long-Term Effects of Weathering on the Aesthetics of Repaired HDG Steel
Will Repairs Made with Zinc-Rich Paint Blend and Match HDG Over Time?
During the first six months to two years after galvanizing, the zinc patina forms, causing the texture and appearance of bright hot-dip galvanized steel to weather from a shiny silver into a uniform matte gray/silver. However, when touch-ups and repairs are required using zinc-rich paint, the repaired area and material may weather differently from the surrounding galvanizing. This means a shiny or bright zinc coating with a similar repair paint color may initially appear uniform when it leaves the plant, but following installation and exposure to the environment, it can weather into an uneven or splotchy appearance. One example is the pedestrian hand railing on the George Washington Bridge between NYC and New Jersey (figure 1). When performing touch-ups and repairs to the freshly galvanized handrails, a shiny zinc-rich paint was used to match the aesthetic qualities of the unweathered zinc coating. However, as you can see, the surrounding galvanized coating lost some of its luster over time, causing the still shiny repaired areas to stand out against the patina.
ASTM A780/A780M describes three acceptable methods to touch-up or repair hot-dip galvanized coatings: zinc-based solder, zinc-rich paint, and thermal spray zinc (zinc metallizing). Zinc-rich paints can economically provide a more uniform aesthetic over time if a matte gray or weathered color repair paint is utilized. Although the initial appearance may contain a mixture of matte gray paint and shiny hot-dip galvanizing, exterior structures will weather and more closely match the repair paint over time. The use of a shiny or silver repair paint will result in a permanent mixed appearance upon natural weathering of the galvanizing. Conversely, interior HDG structures installed in a climate-controlled area will not experience weathering, and the initial appearance is unlikely to weather significantly over time and blending of the initial color is appropriate to achieve uniformity over time.
To maximize the long-term appearance of a galvanized surface, it is important to take these factors into consideration. Will the zinc coating be exposed to atmospheric conditions? If so, will the repair method chosen match the HDG surface now, or after the formation of the patina?
The AGA website contains a directory of hot-dip galvanizing repair material suppliers and instructional videos on touch-up and repair.
© 2023 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.