Good point Bob. I guess my dilemma is choosing the lesser of 2 evils. Its not just paw prints for me, but claw scratches. My theory is that a few nights of water spots may be the sacrifice I need to make to dissuade the cats from returning, so any water spot damage (and claw marks for a cat...
I am having the exact same problem. My research has led me to the following motion-activated water sprinkler system. I am very curious to see whether it would work:
http://www.amazon.com/Contech-CRO101-Scarecrow-Activated-Sprinkler/dp/B000071NUS