Best Finish for Ductile Cast Iron

Question:

I am currently working on a large streetscape redevelopment project that will be using ductile cast iron streetlights. Which of the following processes would be better for long-term protection and a glossy high-quality look on a dark green finish? A traditional low-pressure grit blasting followed by an epoxy primer and polyurethane topcoat or a powder coat finish with an alkaline hot wash, rinse, phosphoric acid etch, rinse, hot zinc phosphate, rinse, hot sealer and finally a 100-micron polyester powder coat electrostatically applied. S. G.

Answer:

I sat looking at your question on my computer monitor a full five minutes before answering it. That is, answering it in a way that addresses your question directly. Both candidate finish systems have limitations. The zinc phosphate pretreated surface will provide better corrosion protection and resistance to film undercutting than the grit blasted one. The epoxy primer/polyurethane enamel will provide better corrosion protection and resistance to undercutting than the polyester powder. In general, the polyurethane system will provide better weather resistance than the polyester, although gloss retention will be similar. I must say in general, because the performance characteristics of both these systems can vary markedly, if they were my street lamps, I would finish them with a zinc phosphate pretreatment, prime with a two-component epoxy and topcoat with a two-component polyurethane. To solve your dilemma, ask the suppliers of the candidate finish systems to provide test results and show you samples of each candidate finish system that have been subjected to corrosion and weathering tests.