Gday Bob
Bondall make a waterproofing product called Silasec that can be mixed with cement to a putty consistency to press into cracks, or with cement and water I think to make a waterproofing cement paint.
I would assume though you would need to do this inside the tank
They also make a product called Epoxy Prep which may be suitable for application from the outside.
Give their Head Office number a ring and ask for Julie Champion, she has good tech knowledge and may have useful advice. Find contact details on website or on one of their product labels.
No affiliation, just bits of product knowledge
Cheers...........Sean, tanks fer dat![]()