Elizabeth 24, I assume you are still in the Tacna area. That "rust" you are talking about, don't know what it is, but I believe it is in the dust that blows around, material gets damp and that seems to be what we end up with. I don't understand why it eventually congregates at folds but here in the Foothills we have the same thing. Folks that let the sunshade on all year and the RV stays here, it seems to roll to the edges.