I'm betting on acid, but I couldn't count out the protein as well. I remember in school someone egg bombing the teacher's parking lot. Our substitute teacher, primarily a science teacher, ran out there with a cup of water and baking soda. After wiping off as much egg as he could, he rinsed the paint then put baking soda. Baking soda is a base, so I take it, it neutralized any leftover acids in the egg on the paint.