I spray some of my horses down several times a day. I've got 4 large horses, 2 of whom are solid black and they love being sprayed down on days like today when it's 100 degrees and sunny.
What the others suggested about the sweat scraper, IMHO that depends on the horse. Mine will immediately go out and roll as soon as I stop spraying them. That works to get rid of the excess water and also gives them a protective coating of mud. If the horse doesn't roll, though, I'd scrape the excess off.