As a general rule, epoxies do not perform well in exterior environments where UV exposure occurs. Exposure to direct sunlight causes the epoxy to break down, which leads to the possibility of moisture intrusion to the substrate. Epoxies which have UV absorbers added, such as SB-112 gain some UV resistance, but not sufficient to last for prolonged UV exposure.
Best results will always be achieved by adding paint or a quality UV protective clear coat to your favorite epoxy. See links below for System Three UV protective coatings.
System Three has a detailed procedure for protecting outdoor wood projects from moisture and damaging UV exposure. See detailed instructions below.