I am a designer in an architectural firm. We have specified exterior ornamental metal in one of our current projects to be both hot-dipped galvanized and powder coated. Is this possible and, if so, is it reasonable? J.R.
It depends upon what you consider reasonable. This design will provide approximately 1,000 hr of accelerated corrosion resistance but may cause problems for the person who powder coats them (see "Outgassing Problems" question). There are several other designs that can provide higher corrosion resistance and can be easier to make. For instance, mild steel with electrocoat primer and powder topcoat or two-coat powder systems (zinc-rich primer and TGIC powder topcoat) can provide the same or higher corrosion resistance to hot-dipped galvanized and powder topcoat. If you really want a high corrosion resistant design, look at aluminum with a chromate conversion pretreatment and a powder topcoat. This design will yield 5,000 hr accelerated corrosion resistance without a problem. If you make these ornamental parts out of pure gold you will never have to worry about corrosion, but they may not stay on the building very long.