I have the same issue with my Paint (an absolute Houdini when it comes to fly masks!) – he reacted badly to cream/lotion sunscreens and I found a product called “My Pony Sunblock” developed by someone in Arizona – figure they should have some ideas. It is a white powder that you apply with makeup type sponge – I put it on usually every evening as my horse boarded and I do not get there before he is turned. I have been using for over 2 years and he has not had any sunburn!!! I tell everyone I see that has a pink skinned nose about it. You can find at myponysunblock.com.