You can clean out the tank with citric acid. I buy it in 20 kg bags, it's great for removing rust and dirt. You mix the acid powder with hot water. I use the 200 litre plastic drums that dairy farmers use. A pressure washer is used to rinse out the tank.
The POR 15 tank seal kit comes with a acid wash , first you clean the tank with the acid wash , then you use the special sealing paint .
