Just use an exterior transparent stain, same thing used on decks, fences and siding, they don't require top coats and advise against using them. Anything exposed to the weather will need re-doing from time to time. Depending on conditions, you should get 5 yrs. and then it's an easy wash and recoat. Many manufacturers to choose from, Benjamin Moore makes Arborcoat, for example.
http://www.benjaminmoore.com/en-us/for-your-home/color-and-finishes
http://www.benjaminmoore.com/en-us/for-your-home/color-and-finishes
