AFter I had mine cleaned, I coated the outside with Chassis Saver. This is a one coat process unlike POR. Do a search and you will find more info on the product. It gives a nice hard finish. I will be doing my chasis later this summer.
I used POR-15, so far so good, If I had the cash at the time I would have had my tank powdercoated. I think any of those heavy duty coating/paints (zero-rust, rhinoliner, powdercoating, etc.) would work well.