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Further Corrosion Resistance for Galvanized Steel Planter Boxes

Regarding best corrosion resistance for galvanized steel planter boxes in a marine environment: boxes were prefabricated of sheet metal and then hot-dip galvanized prior to insertion in the final setting. Now that they are in place, is there a top coating that would improve their resistance to corrosion such as use of epoxy-based spray/paint-on vs powder coating and/or bituminous paint? It would need to be compatible with eventual soil and plants. Boxes are drained, so standing water would not be present. One recommendation was for epoxy primer then bituminous paint. Since the galvanizing was done a few months ago, what preparation would be recommended prior to the application of primer + final coat or single-coat application?

Thanks!

Greg Ginsburg
- Los Angeles, California
2002

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Hi, Greg. At the risk of slight oversimplification, galvanizing comes in two varieties: chromate conversion coated and plain. The chromate conversion coated variety is intended as a stand alone finish; the plain is intended to be painted. Whereas the plain variety will acquire white rust very quickly because there is no coating to prevent it, the conversion coated variety is difficult to paint because the chromate is a great separation layer.

Next time, order the plain type and paint it immediately. This time, try washing it with a scrub brush ⇦this on eBay or Amazon [affil links] and a strong solution of trisodium phosphate ⇦this on eBay or Amazon [affil links] and then applying a self-etching primer made for galvanized coatings. The American Galvanizing Association has a booklet entitled "Duplex Coatings", or something close, describing how to use galvanizing and organic coatings together. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002