I have had many white-nosed horses. Any sunblock lotion will work; I prefer the big bottles of SPF 30+ sport. Apply it every day before the horse goes out.
If the horse is already burned and crusty, it will take a few days to heal. Keep applying the sunscreen (most of them have aloe and skin lotions) and it will improve. Sometimes grazing on certain plants will make the pink skin more sensitive to burn; that’s why some horses burn very little, others much more.
Some horses are more tolerant of the creamy goo than others. Some hate the smell. Most (all in my experience) will learn to accept it. Even foals.
Desitin is good, but tends to attract dirt/sand in my experience. Sunblock rubs in and disappears quickly.