I boil my horns till they get soft. I do 10 or so at a time. After they get soft, take them out of the water and let them air cool untill the dont flex on the walls. Then give them a few taps on the side against someting hard and pull out the core.
Even after the core is out you will need to sand the inside of the horn. (takes the smell out) I use 80 grit bear tex. Then work up in grits till they have the fininish look I want, and seal them with sealer, then clear coat and buff. ( No smell at all after this is complete).
On the out side I start with 80 Grit and go all the way to a 12,000 micro wet sand. And then go to a wet sand vibrate sander to finish,then clear coat three times and buff threw 3 compounds.
Brent