I think for your repairs, Papa knows best!...Sorry! You can go to a boat dealer/shop and buy you a fiber glass repair kit that has colors in it to mix on a piece of clear plastic, like everything is packed in these days. This kit contains the epoxy resin and hardner. Squeeze some of the resin on the clear plastic and then add just a dot of the colors you need to match the color of the horn and mix well. When you get the color match you like, just add the hardner, mix and apply to horn. Fill area's alittle higher than what you need and scrape or sand off!.....NOTE!..By mixing the epoxy on the clear plastic, you can hold the mixed epoxy up over your head and see how the mixture looks after it sets and is smoothed!
Rick