Years ago, I moved into a home with a yucky porcelain tub, much like you describe. I "fixed" it by cleaning it with powdered alum (found in the spice/herb area of the grocery store - used in pickling) on a damp sponge. You can also try powdered alum and lemon juice - mixed into a paste and buffed onto the tub surface. I'd read that hint for cleaning a porcelain tub somewhere.

If your tub is fiberglas, try soaking a cotton towel/paper towel in hot vinegar (or spritz hot vinegar on the towel) and laying it on the tub surface for 10-15 minutes. I read that hint someplace, as well.

-Grainlady

Also keep in mind that if it's a really old iron tub, or the finish has been abused, that gray might be the cast iron showing through worn-thin enamel.

The tub in my house was over 50 years old when it finally dawned on me why the bottom never looked quite clean. Do some checking around for a reliable company, but I highly recommend the epoxy finish (re-finish?).

My clawfoot cleaned up beautifully, except these two grayish spaces on the floor of it. My son pointed out that they were footprints. They are my DH's grandparents and uncles footprints. We think that's cool.

Use Barkeepers Friend. I have a 100 year old clawfoot and it was the only thing that will clean it without removeing more porceline.

we also had a castiron tub where the porcelin finish has worn in some places. To have it professionally re-done was so $$$ (& was told it didn't last forever either) so my son got some stuff from HomeDepot for refinishing the castiron tubs. He has to wear a mask & sand & use a very strong cleaner (& windows open & fans going!) & the brush on the new finish. The result was beautiful ... as long as it lasted!! Wasn't worth all the work & trouble! I'd guess my next alternative would be a made-to-order fiberglass tub/wall insert (which is also $$ but should last longer). To remove the cast iron tub would mean chopping it up or knocking down some walls & I really want to keep that heavy old tub! My other bath tub is steel & I can keep the ?porcelin finish clean by using Tub & Shower spray on the sides after wiping down the sides with the washcloth/tub water after use. Haven't seen any ring at all after a year & that is all I need to do to keep it clean! Before I had to physically get in the tub weekly to clean the sides of soap ring (we have hard water)... now that is 'history'!