Another thing to consider is that the edges along the bottom of the boards may have drip marks on them from the sealers or whatever has been applied to the deck over the years.
I contacted a guy who had an ad in the paper for floor sanding. I asked if he would come over and sand my deck. He said it wouldn't be worth his time and gas money and that I wouldn't want to pay him $400 to do the job.
I probably would have paid that much because there are 2 decks equalling just under 550 sq. feet.
But if someone were to sand it with one of those big floor sanders, would the heads of the screws tear up the sandpaper?