Abstract: A water soluble barrier coating formulation for treating an interior surface of a cargo tank, the formulation comprising a solvent and a rheological additive configured to increase the viscosity of the solvent to allow the substantially liquid formulation to adhere to the interior surface and not drain away under gravity. The formulation is configured to inhibit absorption of a liquid cargo and in particular a hydrocarbon based cargo at the inner surface of the cargo tank so as to eliminate the requirement for detergents and aggressive solvents in order to clean the interior surface following cargo transportation. The time required to clean the cargo tanks is also reduced.