Black marks from protective cover

[ike]

Many months ago I posted a thread re removing black marks which had appeared on the van. None of the suggested/usual cleaning products worked.Eventually we managed to remove them using a cleaner provided by Knowepark. However it did take a great deal of effort. I have since discovered that the marks (black, oily film) are coming from the "protective" cover which we use, albeit infrequently. Has anyone else experienced this problem? It kind of defeats the purpose when the protective cover is causing more problems than it solves. I am still trying to find the purchase receipt so that I can raise the problem with the manufacturer. Knowledge of whether this is a common problem would be very helpful.

Regards ike