I have for the most part adjusted the gap just as a corrective measure when a particular reed is choking or not sounding properly, and I have not noticed pitch changes — but that doesn’t mean it’s not happening, especially since my gap corrections have been mainly on vintage melodicas that I haven’t been tuning.
Here’s where I described the refinishing of my Yamaha in some detail (using Krylon Fusion spray paint and Krylon Clear CoverMax).
There are fingerprint issues, but they’re noticeable only if you look closely. I think a high gloss paint/coating would avoid that issue.
I will probably be refinishing the end pieces of one or more Suzuki (or Samick equivalent) whose metal tray is red or black. An artist friend strongly advises me to not multiply colors. Black is already there on the keyboard. And she also says not to introduce an additional shade of an existing color, since it will be too hard to find (or even recognize) a compatible shade. She works a lot in blacks and reds. Another possibility I’ve considered is having wooden end piece replacements carved. If I can’t make it sound good, maybe I can at least make it look as though it sounds good. I do find that I prefer playing the black P-32D to playing a blue one.
From your avatar, beezer, it looks like you should be ready by next December to decorate your Christmas tree with melodicas. If you celebrate Christmas, that is.