We regret on this occurrence and this is our first time to see on this problem.
Anyhow from pictures, this coil was installed with horizontal position (vertical air flow) and is operating on outdoor without any shade.
THAN coat is Epoxy base, which is very good for interior application or outdoor with some shade but is not recommended to operate by exposure and received the UV directly from sunshine. As the coat will start to fade and finally peel off.