Gord, I think that you have done the best you can. some of these stains in tubs especially the cast iron ones just will not come out.
Are the stains on the sides or on the bottom of the tub?
Try applying some industrial bleach to a piece of bread and let it sit on the stain over night and see what happens. the bread acts like a sponge and keeps the chemical on the area intended to treat.
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