Hi Jan
to the Surface Engineering Forum.
Initially, I would say a zinc thermal spray coating, assuming a relatively rural atmospheric environment and for the external surfaces (steel substrate). This coating should provide protection in a similar way as that of galvanising, giving a galvanic sacrificial coating that does not totally depend on being a 100% barrier.
A bond coat should not be necessary. Surface preparation is a critical stage in the process. the surface needs to be clean, free from corrosion products, dirt and grease. Grit blasting is needed to roughen, increase surface area and chemically activate the surface to provide a good bond to the coating. It is also important that the delay between grit blasting and coating should be as short as possible. Grit blasting needs to be complete for the area to be coated, you should not see any "shiny" areas. Once this is achieved there is no gain in over blasting the surface.
Zinc coatings in rural environments should give protection for many years without the need for any periodic maintenance. The life is dependent on zinc coating thickness. Sealer and paint systems designed for these coatings can also extend life a little and improve appearance. In a mild environment 0.003-0.004" of zinc may give you ~20 years life.
If your environment is marine, industrial or unusually acidic then aluminium coatings may be worth considering. If you are considering coating the tank internal surfaces, then knowing what the tank will contain will be very important in selecting the best coating.
Hope that helps