In my experience Galvanized metal has been extremely hard to properly prep for paint and does not hold paint well. I live in the United States in what is called the "Rust Belt" This is an area of high humidity and lots of snow in the winter requiring salt to be used on the roads. I have found that keeping the underside and wheel wells of the vehicles clean, free of dirt build up is the best long term plan for avoiding rust as long as possible. The surface of smooth powder coat, I am hoping, will make keeping the surfaces clean easier. I will admit that my experience with galvanized metals is limited but I did spec my fire trucks back in the 90's with galvasteel a galvanized metal with the promise of stainless steel results without the cost of stainless. I can tell you that I am in the process of looking to place two of my fire trucks after 14 years or less of service. The mechanics are good the bodies are rusting off of them. Just my experience.