There is a slight oval dip in the middle of my tub that is approximately 6x12 inches. The water settles and has discolored the area. I'm not sure if the tub is cast iron, steel, etc. (not fiberglass). Is there an epoxy that I can use to fill in the dip that won't pop off after a year or two? If not, any recommendations for a DIY (spray paint) fix?