What we need is a rainwater tank with cracks and one or two mini sugru packages.
Pay attention that coloured sugru may colour your fingers and cloth. Idea: wear one-way gloves.
Hint: the left tank ist the one with the crack. The old patch was broken.
Step 1: Clean It
We don't need old patches or dust.
This place of the old patch as seen on the photo needs definitly more cleaning!
Step 2: Apply Sugru
I applied sugru both from outside and inside of the tank.
I should have done more cleaning of the patch area.
Step 3: Check for More Cracks
Step 4: When Work Is Done Wait at Least 24 Hours!
The first patch formed a small bulge over the crack. I think that happend because of the movement of the sides of the crack when applying the inside patches. At one point I decided to ignore it to not risk small cracks inside the patch itself.
After applying your patches wait at least 24 hours before testing the patches.
I waited serveral days as the was no rain to fill the tank. But now there's water in the tank and all looks fine.