I use Swat as a preventative all the time. It works really well to keep gnats away, plus it's medicated and helps heal up spots if they've gotten bloody. The pink stuff lasts longer, but looks funnier. I put it in their ears, around their eyes, in their "arm pits", on their sheaths, and in their groins...anywhere they seem to be getting bit. The clear stuff lasts for 2 or 3 days, the pink longer. It's great stuff.
