I found the top half of a domestic pig skull in the backyard after moving in to a new place, and I decided to paint it!
I left it outside in a hydrogen peroxide bath for about a week to bleach and sterilize, then I left it out to dry. Once that was done, I cleaned out any remaining dirt build-up.
I had been extremely lucky with this skull, as there was no skin or tissue on it to start with, only dirt and dead bugs.