Hi David, sorry you are dealing with a crack in your bathtub. I hope this video will help you get it fixed. Good luck.
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How to repair a cracked bathtub the right way - YouTube
Been there, done that - the right way last a few years, tried a full bottom patch kit recommended by a hardware store, last a few more years, the final try was flex seal, lasted a couple of years.
Just broke down last month and had it replaced. It was the original tub that came with the new house we bought. It cracked within the first year. Developer had it fixed because it was still under warranty. All they did was reglaze it. First time homeowners had no idea or rever experienced this issue in all the places we had lived prior, so thought it was fixed.
Anywayforard to last month. It was an acrylic tub which I guess are know not to last long. So for us to limp along for 24 years with it the contractor was astonished! The reason it was cracking is becaused when installed it was 3'' above the floor without support. It should have been touching the floor! When the surround came out, there to our surprise were the studs!
Nothing but studs. No waterboard or whatever it's called.
We no have a new fiberglass tub and shower that is installed and guaranteed for life. And it wasn't as expensive as I thought it would be.
So my take away is I wish you all the best is fixing your cracked tub. May you have better luck than we did. Just wanted to give our experience.
Try this. https://www.youtube.com/watch?v=X6x1xniYhxk