How do I fix a bath tub low spot?

[Alex Nelson]

Hi Guys,

I am currently renovating a duplex to house hack. In one unit, i noticed that the tub does not drain completely and leaves a pool of water at the bottom of the tub in a low spot. I tried to fix this by filling the low spot with thin set concrete and then sanding smooth. I then refinished the tub with a refinishing kit. However after less than a month of living in this unit, I've noticed that there is already a crack in the finish. I'm afraid this is only going to get worse if I don't fix this better. I don't want to leave it like this long term as it almost certainly will not hold up.

Does anyone have experience fixing this type of issue? Is there something better than thin set that i should use? Is it an issue just with the finish i used? Is this an unfixable problem that I'm better off replacing the tub all together (and have to remove the new tile i just installed as well)? Any advice is welcomed!!