A good polish is swissol, I have used this on my RS another good one is meguirs gold I believe. The swissol is more of a wax than polish so it isn`t liquid and is therefore better. If done right about once every 3 months should be fine to maintain the protection. When you double lock your car, the doors cannot be opened by pulling the handle on the inside, so no-one can try to fish something in and pull the handle or the cable going to it in the door to open it.