For just minor streaking, I just use a cleaner wax by hand.
If the surface is oxidized that may not work.
If it just won’t come clean with dish soap or too much elbow grease for cleaner wax and you don’t want or need to polish it then, for fiberglass and gelcoat with tough stains any product with muriatic acid will do the trick, easily. Most good fiberglass hull cleaner is glorified toilet bowl cleaner. (With hcl/muriatic acid)
I use gel toilet bowl cleaner for fiberglass stains.
Take care to not get it on any raw aluminum or steel surfaces or on concrete.
Concrete is ok if it’s wet and diluted quickly with water. HCL will etch the concrete.
I’m sure there are other good cleaners and suggestions too.
Jimk, I haven’t been a believer, but my enclosed trailer is red and the radius top cap is quite oxidized. I’m gonna try the zep polish on it!