I clean foreclosed homes for a living and we run into some pretty nasty cleaning problems. One of them is cleaning bathtubs with dirt and stains that have been impossible to remove until I found Acid Magic. Acid Magic is made by a company named Certol International and can be found at some stores that sell pool supplies, Ace Hardware, True Value and online. It's used in place of Muriatic Acid. It will not burn the skin, although I have found that it will sting, if you have an open sore. It also has up to 90% less fumes than Muriatic Acid; however, you still do get some fumes that require you to have open ventilation.
When I use this product, I spray it on the area to be cleaned and let it sit while I do the rest of my cleaning. That gives the acid time to work and time for the fumes to dissipate. After letting it sit, I usually only have to use a wet rag to clean out the tub, but occasionally it will take a second application to get all the dirt and grime clean.
You can use Acid Magic on tile, porcelain, brick & concrete and it can be used to clean stains & scum from boat hulls. It also can adjust PH and alkaline in your pool.
Acid Magic works better than any other product I have used for this purpose, and it sure makes my job a lot easier!