I'll bet the Acraglass was just old. How old was it? Either that or not properly mixed, and you would probably know that. The mix is pretty flexible anyway. I have used it with Brownell's dye in it to color repairs to old hard rubber type buttplates, but not horn. I did take the time to be sure the repair area was de-greased well before patching. Your horn might just need cleaning well before the patch?? I have not used anything other than the Brownell's dye, but that works well. A little dab'l do ya'!!
