Fill tank with 1:19 part solution of deox-C to water. As much as possible, leave for as long as it takes to clear the rust, and no longer. Empty tank and rinse with water, then dry with hot air gun, compressed air, or just draining where this not possible. Once dry, fill with fuel immediately, or treat with water displacing product ferrosol if unable to do so. Filling tank with fuel after treatment with ferrosol will not cause any ill effects.