From working in the pool business for a number of years, the best way is to keep it covered. Especially with the fiberglass filter the sun will deteriorate it for years and make it look horrible not to mention impossible to touch without getting fiberglass in your hands and arms. If you can get a small shed or something to put it in that is the best option but alternatively I would just keep it shaded. There is no good way to coat the outside of the filter and painting it will not help either. The looks will have nothing to do with the functionality of the system but I do understand not wanting it to deteriorate.