Theres a product that the car repair guys use on plastic bumpers, don't remember what it's called but it takes out scratches by just barely melting the plastic while you wipe it on. Sounds more severe than it really is, and it works really good on the car plastic. You should definitely try it in an inconspicuous area first as they say.
I've used carburetor cleaner on my black rubrail to take out white marks and small scratches from the trailer guides/docks, works pretty good and probably does the same thing as the above product. It will remove wax from your hull so be careful, I spray it on a rag then wipe down the rubrail. Also tried Brake Cleaner but that doesn't work at all.
