I'll give you the logic behind my advice. These aluminum fuel tanks most likely failure mode appears to be corrosion caused by moisture on the top surface or moisture trapped in the contact points. If you have a tank with no holes and only minor pitting that still has it's sealing integrity, you can clean it and prevent future corrosion by encasing the tank in a layer of fiberglass with a two part epoxy. The fiberglass cloth gives the epoxy resin a matrix to bind to and helps resist cracking of the coating or abrasion at the contact points.
If your tank already has holes or deep corrosion pitting, then a new tank is probably the best way to go. However, I would still apply the fiberglass/epoxy layer to a new tank to prevent future corrosion and abrasion problems like happens with only a two part epoxy paint (scratches expose the bare aluminum).
Now I must go do prep for this hurricane. Good luck to anyone else in its path.
Start with the "experienced boat builder gave me that advice". This a typical internet example of someone making an anonymous statement with no credentials or experience to back it up. It may work and it may not. Doubtful it has been time tested. When buying new tanks for my boat I talked to two experienced tank builders that build to USCG specifications (SP Sheet Metal and Patriot Marine). Neither one mentioned or offered a fiberglass option. Epoxy paint or coal tar were the only options. Aluminum tanks can get wet even without being painted. One told me don't bother to paint the tank - just insulate it properly with glued on nitrile strips. Do a search online and try to find a credible person who recommends fiberglass/epoxy over aluminum tank.
For my forward tank I think it's a mistake to put the old tank back. In the replacement process on my boat there is significant fiberglass work get the tank out and to restore it. If I were to have someone do it (instead of DIY) this is easily $5,000 of labor. If I were to take tank out and repair it (choose your method) it takes labor and materials to repair. A new tank was $1,600.
In the case of the aft tank This is an easy in/out. No fiberglass repair needed. So I could see someone repairing the aft tank. They still have the labor of cleaning the tank up, pressure testing it and then painting or fiberglassing it. A new tank would be $1,300.
Last part is your logic. Aluminum is self protecting (oxidizing) and difficult to bond to. If it's not really clean and painted or powder coated it will oxidize and push the finish away from the surface. I could see it delaminating the fiberglass and trapping water between the fiberglass and the tank. Trapped water against an aluminum tank is a tank killer.
I repaired my aluminum tower on my boat last year. It was sandblasted and primed immediately with AwlGrip Wash Primer. It was then painted with AwlGrip primer and AwlGrip topcoat. It looks fantastic and has held up well so far but it's labor intensive and expensive. If you were to do this on a fuel tank you would have to be sure the tank is really clean and prime it with an aluminum primer.
