User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4081776-20120415014600/@comment-1011548-20120416151233

The Hunting Horns are made out of scales, carapaces and bones like all the other weapons (probably with some metal reinforcement going on inside). So they might not be too fragile.

I've always interpreted using a Whetstone on Hammers and Hunting Horns as cleaning off the accumulated gunk and crap on them (they are getting swung into monsters a lot). Being weighed down by bits of ground, scale and flesh probably makes them more unwieldy, so you do less damage (represented by the sharpness dropping).