The Mr. Clean magic erasers work great on tubs too. You don't really have to scrub, and they leave the tub very shiny!

Is it possible the surface material has worn off and you're seeing the stuff underneath.

I stopped using jetted tubs in my travels after once using Dial Clean Rinse Bath Gel drawing my bathe. When I returned to enjoy the deep tub, the bubbles were black on the top. YUCK!

I recently watched an English show called 'How clean is your house?', they used an emzyme based clothes washing powder to clean a filthy bath, most of the soiling is actually body oils and the emzymes work on those. Said to leave it overnight, I have tried this it does work.

They also cleaned a jacuzzi tub by adding about 6 dishwasher tabs to it. Ran it for a while, can't remember how long but the muck it bought up was unbelievable - the tub shone afterwards.

Doesn't that make you re think using a jet tub when traveling? Shocking how much'muck' grows in there. I wear shower shoes in hotel showers too, just incase. I get tired cleaning my rooms everywhere I go, but housekeeping love me.
Great tips, the dishwasher tabs and enzimes, I also use denture tabs to clean smaller items.

To clean my jacuzzi tub jets I just pour a cupful of bleach into the full tub and run the jets for a few minutes. I got that off the Hints from Heloise page, I think. Big box stores that sell the tubs also sell special cleaner for them, but seems to me that bleachwater would do the trick....

If it is fiberglass, use SoftScrub GEL. Put it on, let it sit for 5 - 10 mins, scrub with a scotchbrite side of spounge, rinse well. You won't believe it!

I love Lysol basin and tube cleaner. Actually liked their "Direct" product better but they stopped making it a couple years ago. Every once in a while I also pour some Clorox Clean up Liquid in it and add water.

You could try using diluted ammonia, that often works to lift tough stains and should be safe on the bathtub surface.

Or try baking soda and water.

Of course you could buy a commercial bathtub cleaning product, which should help to the trick as well, if you in fact are looking at built up dirt, rather than permanent damage to your tub.

Good luck!

Here is a link that might be useful: Bathroom-cleaning.com - Cleaning Bathtubs