This isn't the first time we've faced this problem. We've had complaints from many of our readers on this very subject. One frustrated homeowner went so far as to compare the condition of her fiberglass tub and shower to something that resembled nuclear waste. Another was so thoroughly disgusted by the surface scum build-up that she considered converting the bathroom into a vegetable garden!
Now, if this even slightly resembles the problem you have there is hope. Try making a paste out of 1/2 cup of turpentine and two tablespoons salt. Wearing rubber gloves, rub the paste onto the stained or marred areas and scrub with a nylon brush. Thoroughly rinse the entire area with warm water and wipe dry. And, you may wish to consider a water softener to eliminate these hardwater deposits in the future.