Tow ball rust protection
 

Now I realise that this is a little out of the ordinary but I use my car for caravaning. I've just changed my van for one with a damper type hitch that clamps onto the tow ball. Despite my initial doubts, it does seem to work. The problem is the tow ball needs to be absolutely clean so it goes rusty when not in use. Any ideas how I can protect it? The best I can come up with is Waxoyl and clean it everytime I tow. Obviously anything greasy will mean the stabiliser doesn't work.