Originally Posted by mwilson
The plumber found a crack in the end, apparently extending down from the overflow drain, which is the cause of a leak. It only leaks when the tub is filled, not when I shower.
I'd get another opinion.
I have seen chips in bathtub enamel, and I have seen scratches in bathtub enamel, but I have never seen a "crack" in bathtub enamel. Also, the only evidence that the crack would be of concern is seeing a reddish brown rust forming along that "crack". Unless you see that rust, then the crack doesn't extend all the way through the porcelain enamel to the steel. If the steel of the tub is rusting (and it WILL rust if exposed to moisture), you should see a reddish brown discolourantion along that crack.
Also, steel bathtubs don't crack! Think about it. In order to stamp a sheet of steel into as radical a shape as a bathtub, you can't use a hard or brittle steel. You need to use a softer steel that can be bent and stretched without breaking. That this same steel would then crack when there's virtually no stress on it is kinda hard to understand.
Unless you can see that crack with your own beady eyes, what you probably have is just a leaking overflow washer. This is a bevelled rubber washer that goes between the back of the bathtub and the overflow pipe that connects to the drain pipe from the tub. Then, the flow from both pipes goes into a p-trap. I've had to replace one of those myself even, and you can change them from inside the tub.
You can buy a replacement bevelled washer for the overflow drain on your bathtub at any hardware store. It's not often that these rubber washers leak, but compared to cracked steel bathtubs, it's as common as a barking dog. That is, it's not common to have to replace an overflow washer, but I've never even heard of a bathtub cracking until now.
Also, the reason why you only have a leak when you bathe could very well be that it's only when you bathe that you get water flowing out the overflow spout on the tub (and leaking past the beveled washer).
Here, do this:
Clean the tub surface around the overflow drain, and tape some plastic over the tub overflow drain. Slit a plastic bag open and tape it over the overflow all the way around the perimeter with wide, waterproof packaging tape. (Any "Scotchtape-type tape" with an acrylic or solvent based pressure sensitive adhesive will do.) (Open your yellow pages to "adhesives" and you should find sales agencies for LePages, 3M, Henkel, Dow Corning and others, and every one of them will sell you a roll of cheap waterproof tape.)
NOW fill the tub with water and see if there's any water leaks.
If no water leaks...,
...then it's leaking past your beveled overflow drain washer, not through any crack.
If it still leaks...,
... then clean the area around that crack, and apply your tape over that crack, all the way up to the overflow drain. (You can remove the chrome cover over the overflow drain and run the tape right up under the chrome cover.)
Even if your tub actually is cracked, there's absolutely no reason why a roll of waterproof tape won't allow you to continue using that tub for many more years. Do try to get a transparent tape, tho, to make the repair less visible.