I've seen on Amazon and eBay how you can buy water filters that you connect between your outside tap and the hose. It seems like a useful tool for people that live in hard water areas (like me) and the main selling point appears to be you don't have those dreaded stains on your car if you wash it without drying afterwards. The ones that I've seen cost around £80-£100. I know that drying the car is clearly a cheaper way to avoid the problem but I was just wondering if anyone's ever bothered to use these water filters? Or what they contain to justify such a high price?