Any kind of harp becomes louder the moment it is immersed in water.
Removing 'gunk' takes longer than this.
I repeat, it is because the water temporarily fills up any gaps,
especially the clearances between the reeds and slots.
It's more likely that cleaning out the 'gunk' would make the harp
leakier and therefore quieter.
I never really bother cleaning diatonics unless I happen to have the
thing apart and I can see layers of muck flaking away and threatening to
end up in my mouth or get caught in a reed.
If a wooden harp is played frequently the comb should remain moist
enough not to shrink and require dampening. The 'gunk', likewise,
usually only starts to flake off if the harp is left to dry out for long
enough.
Cleaning diatonics? Nice for people with that much time to spare......
RD