Recently I was visiting _____ (name has been blanked to protect me from harm) and discovered a nightmare in their bathroom... their bathtub was brown. I mean, I can make a mess but seriously, a brown bathtub?? So I fixed it for them.
C's solution for nasty nasty tub-
Make a paste out of baking soda and water. Put a large quantity of the paste onto a rag and SCRUB! Normal soap scum will come up like butter, harder soap scum might take a small amount of elbow grease. I have found when stains get that bad, 409 won't help you, bleach is no good, and small scrubber on the back of a sponge, please you'll gum it up in 10 seconds.
Scrub tub, rinse with water, or clean out your rag and wipe the tub back down real quick.
And when you get done, not only will your tub be clean and sanitized, BUT IT WILL SHINE!