I’ve got an old clawfoot bathtub that just will not get clean.
I’ve tried bleach, clorox type cleaners, x-14, damn near everything.
It seems to hold this grayish dirt-like matter on the bottom of the tub and it never looks quite clean. The sides and ends look fine.
It there an old secret to getting this stuff out? Any ideas for a product I could try?
Try a mineral-removing product containing phosphoric acid, such as Lime-Away or CLR.
Try that Zap stuff. I’ve heard from some people that it works pretty well.
Depends how old the bathtub is. Remember, the white surface is porcelean. After years of scrubbing, the porcelean wears thin and you might be seeing the cast iron underneath. Since the bottom of the tub tends to get scrubbed the most, this is where it wears out first. No amount of cleaning will get rid of that.
If it’s installed, you can find people who will epoxy it(which holds out pretty well). If it’s in your garage, you might be able to find someone who can strip the porcelean off and re-coat it. The expense is more than a new clawfoot bathtub, but if it’s an antique that you want to keep it’s worth it.
b.
You can buy the stuff that Billy mentions & do it yourself at a hardware store.
Why not have it refinished by a professional. It will look like a new tub. Look in your phone book for bathtub refinishing. It doesn’t cost too much. 
Before going to any of that trouble, be sure to try one of the acidic products that Fear Itself suggested. If the stuff is just built up carbonates or sulfates, they’ll be no need to resurface the porcelin.
Don’t let bleach sit on the surface overnight–it’ll eat off the remaining porcelain and give you a very nice pattern of rust spots.
Saith the Sad Voice of Experience. 
AH HA! Something I haven’t tried. I’ll pick some up tomorrow and let you all know how it works.
I don’t want to spend the money fixing up the tub because the house is for sale at the moment… I’ll let the new people deal with it.
But I would like it to be somewhat clean looking for both myself and anyone looking at the house.
Well, CLR helped a little, but not much. In fact it hardly helped the little rust line that runs under the fauset.
I did manage to get a small splash of it on my arm and now it itches so all is not lost. heh heh.
I got a good look at the gray areas though and it’s not worn down. I got out my Dremmel and buffed out a little spot just to see. That area is perfect white now. But I’m not going to dremmel the entire tub. LOL
I’ll see if I can find the zap stuff Mr Moto mentioned.
Any other ideas?