I wonder if that tub has been refinished?

maybe try a wax on a spot and coat it. Don't really know.

I dunno, but I'd be suspicious that it's been coated too. It may have been a "bad color for resale" or just beat up.

I'd suspect it's been coated but after 70 years or so, the original porcelain would probably look pretty lousy anyway. Scrubbing on it will just make it worse. Recoat it again or buy a new tub.

The amount of scratching in that one picture tells the whole tale. Scrubbing this for hours and wondering why it's giving off powder is like sanding a piece of wood and wondering why it makes sawdust.

If this isn't the only tub in the house you may just want to remove it altogether.

The tub has been resurfaced. Resurfacing never works properly and in fact is only ever recommended (by ethical people at any rate) purely for looks in a seldom used tub. It is NOT anything you did to it.

Replace the tub. I have a 60 year old cast iron/porcelain tub that has worn through to the cast iron, after over half a century it has more than done its job. Time for a new one.

i am not challenging the advice that's been given, but we have a claw foot tub that was so worn down you could see the iron right through - black streaks up and down the front below the spigot. we had it re-glazed for about $250. just the inside was finished as its sunken into a platform built up around it. exterior not visible. 7 years later it still looks nearly new. the texture of the ridges are still there, which give it some character as an old tub, but the glaze is still shiny white. its used daily by my husband and i. i only clean it with bon ami.

i am curious though, how successful is re-glazing if we were ever to need it?