Things You'll Need
Resin and ketone repair solution
Water tanks are often made of stainless steel because the metal is resistant to rust and doesn't corrode as easily. However, the steel isn't generally more resistant to other damage and can be breached like any other steel. A leaking stainless steel water tank needs immediate repair to prevent further water loss and protect the area around the tank from damage.
Drain the tank of any remaining water so you have full access to the tank's interior without worrying about its contents.
Clean the inside and the outside of the tank with a pressure washer to remove any dirt that could interfere with the repair process.
Use a shop vacuum to suck up any excess water in the tank and wait for it to fully dry out before continuing.
Remove the water connection fillings to the tank and plug the holes with rags. This keeps the holes from becoming closed due to the repair application.
Spray a resin repair compound and a ketone solution over the leaking area inside the tank with a garden sprayer. The compound comes premixed; but if the solution is older and has started to harden due to evaporation, pour a small amount of acetone into the container to soften it again as this will dissolve the hardening resins. Allow the tank to dry for three days. The ketone solution evaporates during that time, and the resins bond to the tank to plug the leak.
Avoid inhaling the ketone/resin solution and keep the area well-ventilated while you work. Wear safety gloves and a respirator to prevent respiratory problems.