I learned this from my daughter's cleaning lady: Making sure that shower or tub is dry scrub with a dry SOS Pad, most of the scum will come off. Then wet shower & pad and srub off whatever residue is left. Works great on glass shower door, too, and no it doesn't leave scratches. :-D
P.S. This works great on tile, don't know what it would do to a fiberglass tub.