You're well aware of the damage the sun can do to your own skin, but don't forget that ultraviolet rays put your horse's health at risk, too. In addition to getting sunburned on exposed light skin, particularly around the muzzle, horses can be harmed by the sun in other ways.
Squamous cell carcinomas are tumors that form as the result of extended exposure to sunlight. They tend to form on pink skin around the eyes or on the third eyelid of light-colored horses.
Photosensitization occurs when sunlight triggers chemical reactions in the blood just below the skin's surface. The result is a "burning" reaction that leads to painful scabbing on pink skin under white markings.
Protecting your horse from sunlight requires diligence and some specialized gear. A good place to start is a UV-blocking fly mask, which will protect? your horse's face and eyes against both insects and sunlight. Some fly masks come with a nose-covering flap, or you can sew one on yourself to protect a pink nose. Otherwise, you can coat the muzzle area in a thick zinc oxide cream similar to formulations used by lifeguards. It will be messy, but effective.
To protect white body and leg markings, you may want to try one of the many grooming products and even fly sprays that contain sunscreen. But your best bet may be to invest in UV-blocking wraps and sheets. Otherwise, you'll need to keep your horse in a shaded area during daylight hours.