I'm not worried that the tank won't be sealed when I'm through. What concerns me is that there's been so much leakage through the years that there are big hollows under the tank. When the weight of cold water rushes in and fills the possibly warm tank in a few minutes, cracks might develop and a previously sealed tank can instantly fail. That's what happened last year. I had it sealed, but cracks developed, perhaps with freezing temperatures at night, followed by hot days. So, if this doesn't work, then I'll live with it the way it is. I found a really bad leak today but forgot to take a picture of it. If you look at yesterday's photo, it's at the base of where the shovel is leaning, running toward the camera, sort of. I patched it today, but don't have a lot of confidence that the weight of water pressure won't cause shifting below the tank. Nothing I can do about that, except put a new patch on every year.
On the above today's photo, if you enlarge it, the big crack is just below where I quit coating on the far left. I patched it with gray Thoroseal mixed with concrete adhesive. I'll eventually give it a couple coats of Drylok too. The floor of the tank is very thick, and has lots of steel in it. The walls are thinner and have mesh in. BUT where there were boulders we couldn't get out, I stuccoed over them with no mesh. They were bedrock. I never thought they would become a problem, but that bedrock is caliche and does dissolve with enough water over a long time, apparently.