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I would be suprised if you could find a plating shop with zinc tank big enough to plate a whole frame. Plus zinc is a poor coating to stop rust. I worked for a company the builds packaging machines. In early days (mid-80's) we zinc plated parts. But from feed back from customers the zinc didn't really provide much rust protection. We switched to nickel plating and it was the answer.
Galvanizing is good at stopping rust but it's ugly to look at. Every tapped hole has to be re-tapped. We built a number of machines for the dairy industry. They had to be heavy wash down and we galvanized the frames. Outside shop did it for us. Then in the mid 90's we switched to stainless frames and parts for all wash down machines. No more galvanizing.