Hello Carola, I don’t think that a nano coating would help since the issue is the accumilation of moisture that affects the performance. blowing the moisture out frequently would help. There has been some other suggestions like an air pump that pulls the moisture out.
here is the link. https://melodicaworld.com/whats-a-melodica/…
scroll down to “How do I dry out my melodica?”
Standing your melodica upright and letting the moisture accumalate then releasing the moisture while in the upright position works too.
Melodica-me