Can anybody suggest how I can stop the black streaks that appear down the bodywork panels of our Chausson when it has been left for a couple of weeks during which it has rained. It is stored outside, slightly front down and the worst streaks are caused by run off from the top that comes down between and above the doors and the windscreen. I go on the roof and wash it all the over using protection stuff prior to a winter, but that hasn't stopped it. It also runs down from the corner of some of the windows and in certain places down from the black sealant under the beading at the top of the body panels. It washes off quite easily, provided you use something besides a water and a brush/hose pipe. But why does it keep appearing?