Maybe the best black streak remover ever.
Ugh! black streaks on my RV.
For those of you that own a boat or RV, you know what I am talking about - those black streaks that show up after a rainy day... they can be almost impossible to get off - especially fiberglass. Well I tried some Tuff Stuff Foaming Cleaner one day, and to my surprise, the black streaks came right off. This stuff works great on fiberglass for both boats and RVs, as well as painted metal surfaces.
The makers of Tuff Stuff Foaming Cleaner indicate it is safe for these surfaces, but I have found that it seems to remove wax. However, if the wax was doing it's job, then you would not have impossible-to-remove black streaks to begin with... so just plan on waxing your boat/RV when you are done removing the black streaks.
You can buy this stuff virtually everywhere; Wal-Mart, Auto parts stores, and it's even probably at your local grocery store.