There are these oval stains all over the floor of the bathtub and I am guessing they are left over from nonslip appliqués that were removed before I moved in. They are rough to the touch unlike the rest of the bathtub surface. I did some research on this before and basically concluded that there was nothing I could do to remove them (correct me if I'm wrong!) and the best I could do was to glaze the tub. For reasons not worth getting into, I'm not going to glaze the tub, but I would like to improve the look of these marks, if I can. They are dirty-looking, is there anything I can do to make them lighter? Such as soak them in bleach? I've tried scrubbing them with SoftScrub and Comet and neither works. Please help, they look terrible!