Paint or powder will both release from the substrate from a scratch if the scratch is not covered and eroding contaminates are allowed to attack the substrate (metal)
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powder is generally more elastic than paint when fully cured, and when applied to properly prepared metal, or alloys usually gets better mechanical adhesion as well. During the cure cycle when baked at 400* the powder then turns into a liquid and "flows out". which means it literally moves across the part to fill in every microscopic crack.

the reason any coating flakes, paint, powder, chrome, etc is because they was a scratch on the surface, and moisture, air, salt, etc get underneath the coating and erode the bond to the substrate.

However PPG, Dupont, Sherwin Williams, who all manufacture both paint, and powder have in my experience always recommended powder for anything automotive where applicable. By these company's who spend millions in research, it is their common consensus that powder is cheaper in materials, labor, and process than paint. More durable, and has a considerable amount more salt spray resistance (our process is 5,000 hours salt spray resistance minimum)
salt spray test is where they take a powder coated part, put a scratch in it an spray it with salt water to simulate the ocean. depending on the company's process it could take from between 1,000 - 7,500 hours of salt spray befor it damages the coating.
this these is in regards to the coating, and not the bond between the substrate, and coating.

we do some werk for boats (not a ton of them in Columbus) that go to the ocean, and the great lakes. I have never gotten negative feed back from any of them. I even have a few who I see at the beginning of the season that are having chrome parts powder coated in stead of buying chrome ever 2-3 years.

thanks.... now I sound like a geek with all this fancy talk, and $5 words