Pasithea

If you have a porcelain tub, get the works toilet bowl cleaner. The kind that comes in a round bottle, not the flatter bottle. It contains sulphuric acid or some such, and gets everything off. Use in a well ventilate area though, and not directly on faucets and metal drains as it discolours them greatly. Oh, and never use on fibreglass tubs, it tends to melt the surface into a gel, it does re-harden into interestingly shaped hills and valleys with the stains now imbedded (can you tell I've made that mistake?).