I will let you on a secret.
This btw has been proven for humans so it will be the same for horses using suncreen. You have to apply sunscreen every 2-3 hours to be effective even if it has a higher SPF. They have found out that SPF 30 is just as effective as the higher SPF's out there and you end up just paying more money in the end. Even waterproof sunblock wears off.
If you want something that last longer I would get a jar of Desitin.The stuff for diaper rash. This stuff is thick and it has zinc in it and that's what you really want. It blocks the sun and it's very hard to get off even when they try to rub it off. I use it and lasts two days for my horse and she has a bald face and burns easily! I honestly would try it Desitin Diaper Rash Paste, Maximum Strength Original Paste, 16 oz