Cleaning an old stained fiberglass tub/wall

My DD bought a house that was a rental for years. Oh my gosh! the projects abound. She tried bleach and all sorts of commercial sprays to get the fiberglass tub and wall clean. Nothing touched it. I went on line researching this and found a site that suggested 1 qt turpentine and 2-3 T table salt. Scrub w/ a plastic bristle. Wipe off with paper towels. Air dry and then rinse w/ a soapy solution. Air dry again and apply car wax. Well, it worked wonderfully! We have not applied to car wax yet but the tub is white for the first time since she bought it a year ago.