In two years there has not been a lot of "new" progress in coatings. There is a lot of stuff in the labs that is "near" market, but it has been there fore quite some time. Most of the industrial and aerospace work has been elimination of chromates and reduction of Volatile Organic Compounds (VOCs).
Any coating is either a barrier coating that protects by keeping the water off the steel or an anti-corrosive coating that uses galvanic properties to protect the steel.
For most aerospace we use an epoxy anti-corrosive primer with a good quality barrier polyurethane top coating.
If I was to do the most perfect coating available at this time I would:
Hot dip galvanize
Epoxy prime with a chromated primer meeting MIL-PRF-23377 or MIL-PRF-85582
Polyurethane top coat with a paint meeting MIL-PRF-85285.
For you, there are a number of good systems given so far. First you need to remove all the loose paint and remove or kill all the active rust spots.
If you have a lot of blistered paint, blast it off with a medium abrasive such a walnut shell or Type III or Type V plastic media. The blasting house should know what these are. Sand is much too abrasive and will pit the surface and remove any remaining anti-corrosive coatings.
Or, use a paint stripper such as zip-strip or aircraft stripper that contains methylene chloride. Do it out side and wearer nitrial gloves. Do not use any alkaline strippers on any tanks, especially aluminum ones.
Wash the surface with soap and water and use an abrasive pad such as red scotchbright to remove any remaining contamination from the surface.
Then use a good quality zinc rich primer that will give some galvanic anti-corrosive properties.
If your zinc rich coating is an epoxy, you must remember that epoxy's do not like direct sun. The UV rays will attack the resin binder and you may end up with chalking of the coating. Now if you keep your tanks out of the sun most of the time in a garage or under a tarp, this should not be a major problem. But if they live on a boat and see sun a lot, you will need to protect the epoxy.
So, put on a good polyurethane top coat. This is also good if you want a specific color other then primer.
The Mil-Specs primer and paint listed above can be had from most aircraft refinisher, but they are mutli-component coatings and are expensive. Expect to pay at least 60-100 per quart. If you are in an area where there is a lot of aerospace work, look for industrial paint shops and see if they will do your tanks for you when they do a large batch for someone else. But you might end up with a Navy gray or Army green unless you want to pay for a specific color
