I don't know if I understand correctly what is going on with the horn, but if there are bits of the surface layer that are getting detached and catching in your clothing and such, I'd use auto body grade fiberglass (like Bondo, which is probably the best brand for the money).
I've worked quite a bit with this stuff and the beauty of it is that it's transparent, can be made as thick or thin as you like (thick like jello or as liquid as paint - depending on the amount of hardener you put and how fast you work with it) and covers surfaces with a shiny, glass-like coat.
Of course, I've mostly applied it in automotive-only applications, but the principle is the same. Once it would soak into all the pores & fibers of that horn and then harden, I bet you the horn would outlive all of our great-great grand-children, lol - and become a symbolic antique some day.
Basically, fiberglass resin is like epoxy... but in liquid & transparent (clear) form. You apply it evenly in thin coats with a normal paintbrush like paint, then you wash your paintbrush in acetone. Once it hardens, it becomes like metal, plus it's totally waterproof and will never rot. Personally, I've had a lot of success with it...