If you can't spray up at 90', does it work well if spraying down. Like spraying deck boards.

It will work for deck boards, it has a tip inside the reservoir that bends any direction you need to aim, so if your aiming down you can position the feed line to feed until it's empty in the direction you are aiming. I would still suggest rolling or brushing in on the horizontal surfaces though, to ensure the stain your using performs the best with water over the years.

Spraying downwards should not be a problem, besides the occasional drip from the nozzle if you point the sprayer downwards too far.