Problems with Americast Princeton tub

I have an Americast Princeton tub that was installed by a home remodeller in the year 2000 as part of a full bathroom remodel. There are a couple of problems with the tub, and I'm hoping to find out what possible causes for these problems might be and how to go about fixing them.

First, there is a low spot in the tub, starting almost right at the drain. That is, part of the tub is actually lower than the drain itself. The drain is on the left hand side of the tub. The low spot is oval, about 15"x6", starting within about an inch of the right hand edge of the drain. The 15" dimension runs lengthwise up the tub to the right. The problem is, a "ring" has appeared around this low spot. Actually, initially I didn't realize that there was a low spot. I just noticed that there was this "ring" that I couldn't get out. Upon closer inspection, I found that there is a puddle is left in the tub after use, and over time it has left a "ring" of hardwater deposits. I have tried many products, but none of them have helped to remove the ring. Also, there is some rusting beginning inside the ring. What could have caused this low spot? Would this have been a defect in the manufacturing of the tub? Or is it more likely that there was a problem with the installation that caused this? (I've never installed a tub myself, so I don't know what such a problem would be.) More importantly, how do I go about fixing it? Even if I were to find a method for cleaning off the ring, if I don't eliminate the puddling, the ring will just come back, so I really want to fix the low spot. The ring looks ugly, and I figure that eventually the rusting will be a real problem, but I suspect that that may be a ways off. I've thought about several possible methods for dealing with this. First, I could have the tub refinished, but presumably that wouldn't do anything to eliminate the low spot, so the puddling would continue, and I'd be back in the same boat in the near future. Second, I could have a liner installed. Could I get the low spot fixed as part of that process? Third, I could just install a new tub. In some ways, that's my preferred solution; in other's its not. It seems like the cleanest fix. But of course there is lots of other work that would be necessary, like removing and replacing tile and cement board.

Does anyone have any good advice on an approach to fix this puddling problem?

My second problem with the tub is that there's a probably 1/2" chip out of the tub's finish on the back side of the left end of the tub. I am assured that nothing heavy or metal was ever dropped in the tub. This is the kid's tub, so who knows. But on the other hand, as I recall, the previous tub had a big chip out of it in about the same spot. I have no idea what the old tub brand was. It was probably the builder-installed tub, installed in 1979. Could there be something about the installation that could cause a chip to occur in the same spot on multiple tubs? I sure wouldn't want to fix the first problem with a new tub, only to have a chip appear again.

Thanks in advance for any help/ideas on fixing these problems.

I think the only good fix to this is to replace the tub. I'd call the manufacturer and seek their opinion. If it is determined to be a manufacturing defect, they'll likely replace the tub, but that does not cover any labor or the tile (as far as I know). The rust is the beginning of the end. For max longevity, you can't beat a cast iron tub.

If the floor is not flat, it's possible that applies some torque to the whole tub. You want either have a tub that is stiff enough to support itself (CI), or provide good support underneath it for everything else. Plus, the tub's top surface must be level in both directions to drain well and prevent water from pooling places.