. It appears as if the tub is has one of those non-slip surfaces. This is an old thread that addresses this issue

http://ths.gardenweb.com/forums/load/cleaning/msg0621310831905.html

Wow, good to know I'm not alone here. I think I'll try the Dawn/Comet combination, as I have both of these on hand. Thanks for your help!

Easy Off oven cleaner in the blue can (less fumes) works great... I use to use it all the time because the regular cleaners just did not cut thru the soap scum build up on the non slip bottom of the shower. I honestly thought it would never be clean or look good again. I read on a message board about using the oven cleaner and tried it with great success. Now as a regular tub/shower cleaner I use half and half mixture of regular strength Dawn dishwashing liquid and vinegar in a spray bottle.

Mr. Clean magic eraser, is the best thing II have found for the, non slip finish in the tub!

Red

I had the same problem when we bought our house. The tub was not in bad shape but the bottom of the tub (non-slip section) was awful. I tried a few suggestions in this forum to no avail. I decided to use Bon ami non abrasive scouring powder with a Mr. Clean eraser. WALA it worked beautifully. Now granted my tub I gather had never been scrubbed so it took some scrubbing and a couple of Mr. Clean erasers! I have used Bon ami on my stainless steel pots for years and I have had them for over 30 years and still look new. I figured if it worked to take burn stains off my pot it should work on the tub. You can also use bon ami on your glass cook top, works wonders!

I had this problem with my last home and used bleach regularly which worked for me to get rid of the stains.

Mr Clean Magic Eraser, and for that much buildup add to it some Bon Ami... later maintenance just use the magic eraser.