I want to stress the importance of thoroughly rinsing all baking soda residue out of your melodica. Otherwise, the residue will harden into a coating, as you can see if drops of moisture fall on hardwood floor or other polished surface. Whatever water you can shake out of your melodica at the end should be clear and not leave water spots. If spots are still appearing (which happens within 10-15 minutes) when you shake it out, then the rinsing process is not complete. Obviously you don’t want any residue to remain, especially on the reeds. It’s probably a good idea to include some toothbrush work on the reed plates while rinsing.
I guess is kind of a “duh!” thread in which nothing ingenious is offered and the warnings seem obvious, but I have not seen a detailed discussion of deodorizing melodicae. The good news is that it’s easy to do, though it requires care, and it makes a big aesthetic difference with some of these old instruments. Also, if you were selling a vintage melodica, it would be nice to be able to say that it has been deodorized. One other thing I should have mentioned is that the melodica case should also be deoderized. This is easily done with a cloth and rubbing alcohol (or with a baking soda solution); the result you want is the “new car” vinyl smell, which the mint condition old stock Hohner Pianos have. Finally, if you’re a collector who lives in a very humid climate, it might be best to store your melodicas in a ventilated container with a box of baking soda.