I've used Desitin Diaper Rash ointment or Zinc Oxide ointment for sunburnt noses and protect the nose from more sunburning. It is a very thick ointment so I stays on longer than sunscreen lotions. It is safe for use on horses noses. Life guards at swimming pools put it on their noses to prevent sunburn.
For prevention, a generic, waterproof, baby SPF65 sunscreen every few days does the trick (even without a mask) for our almost all white, very pink skinned mare when she decides to spend days in the blazing summer sun.