Black run marks

Just been trying out an idea for stopping black run marks down the side of the van. I purchased from my local caravan/camping shop a few meters of awning rail tape. It has the beading plus about 25mm of tape which I guess is for repairing awnings or making your own covers etc. feed this in the rail when parked up in store and the rain will come off the caravan roof and over the tape generally clear of the van wall, this reducing or even eliminating run marks. The other up side to this the tape costs approx £1.50 a meter. I fit about 2 meters at the corners of the van, where the runs seem to be worst. Not a total solution, but try it it works for me and it takes little time to fit at the end of your trip out.