I run a powdercoating operation as part of a production plant. If you have clean HRPO steel, make sure to get all the copper deposits off the welds before coating. We run everything through a 5 stage phosphate washer. Cleaner, rinse, coater, rinse, RO halo. But if you're running anything with mill scale, you want to sandblast first. Anything that can flake off will over time, and take the powder coat with it. And once there's a chip, there's room for moisture to start to rust from the inside out. There's powdercoating for a bunch of levels of salt spray resistance. Ours will hold up to 1000+ hours. There's some, like office furniture that only requires 200 hours. Thickness of spray matters too. We run 2 to 2.5 mills. Anything more doesn't flow out right in the oven. Anything less won't have the durability. There's a ton of factors in powdercoating that determines a great or a shit product. Go to a good shop. Cutting dollars going to the most lowbuck shop will get you a lowbuck job like getting a vehicle sprayed at maaco.