Straight CLR, plus a little elbow grease works on the
fiberglass on semi's. For the really difficult spots use the CLR with 0000 steel wool. A quick wax job, and it looks like new.
Remember, by elbow grease I don't mean pressure, just time. Moderate pressure is all that's required with the steel wool. Start
light, and work up the pressure, I think you'll be surprised how easily it comes off if you started with a good surface.