Time for an UPDATE:
Next month it will have been two years since I refinished my Yamaha P-32D in black. In answer to Lowboy’s question, the paint job has not stood the test of time, as these photos will show. It is only very recently that I began to notice the original blue showing through in strategic locations, namely where my left hand grips and slides along the melodica and by the mouthpiece hole. I play this particular P-32 regularly and almost never with a mouthpiece. The large spot below and to the right of the mouthpiece entry is where the end rests on my typically unshaven chin; the smaller patch is typically in contact with my upper lip.
These photos make the situation look worse than it actually is, as the melodica still has its nice black look to the eyes of the player and from a distance. I could happily continue playing it as is for a few months, but I have decided to sand and refinish. This time I will sand enough to prep for a primer (which I did not use last time), and I will use a few more coats of the Krylon Satin paint and of clear gloss, with special attention to the places where the blue is now showing through.
Meanwhile, I have designated a P-25E (predecessor, made in Japan rather than Indonesia, of the current P-25F) for refinishing in a different color. Rust-Oleum American Accents colonial red gloss, I’m thinking, or Rust-Oleum Ultra Cover Satin walnut or espresso. The prospect of refinishing again after two years doesn’t bother me at all, but I’m open to suggestions for how to get a harder, more durable and relatively permanent finish. I know it can be done, and I’m looking at what some do with guitars.