Blurb; Use an oil based paint. Use an exterior primer sealer first. You could add some linseed oil but will have to wait longer for it to soak in and dry. After the primer has dried put a second coat of gloss exterior oil based paint, as indicated white is a good color. (You might have to search for it. The powers that be don’t like oil paints, bad for the enviornment etc.)
If you were in a really wet climate I might suggest weep holes on the bottom (normally in masonry walls) but its dry most of the year. Just make certain you do a good job painting the cut to size 2x6 frame surfaces (including the end grain) before you assemble. Remember that wood does expand and move and leave some space between your 1” x 8” decking planks. (Use galvanized screws to fasten. Less likely to rust off or back out. )
Good luck!
Thanks so much for the tips Pete. I never would have thought to paint the boards and ends before construction! It looks like the consensus is white or light paint, preferably with UV protection and oil-based. Quick followup: You mention linseed oil. Could I first apply a coat (or two?) of linseed oil by itself, followed by the paint? Or is that overkill? I’ve been meaning to buy a canister of linseed oil anyhow, so the added cost is not a factor. Thanks again.