What kind of finish is on it now? Are you looking to change the finish? or just freshen up the current finish?
My dad and I just did up his recently, we started by blasting most of the old peeling paint off with a powerwasher, and then gave it a quick smoothing with a belt sander. After that you should be able to apply any type of finish to it.
If it has a varnish (Shiny) finish, it could be a lot more work, as that stuff is really hard, However a belt sander should do the trick, just might have to do it twice, once with a coarse grade paper and then once with a finer one.
On another note, I just helped my cousin put in a deck a few weeks ago, and he used this Plastic type of material, that looks like stained wood!! Apparently it will not fade for 25 years guarenteed
This is all I would ever use now after finding such a product, refinishing anything sucks.
If you would like a hand with your deck let me know I love doing stuff like that, I dont have much for wood working tools, however I could probably go over to my old man's place and borrow some if need be.