Protecting an aluminum fuel tank from corrosion

[Pauls]

I'm seconding Uncivilized's comments. You need good prep, which will be mechanical cleaning, then acid etch, then alodine, then zinc chromate, then a top coat.

It is important to understand that the zinc chromate is a protectant for the aluminum. It is NOT a topcoat except in very well protected areas, like inside an aircraft structure. So you topcoat the zinc chromate with something tough. Coal tar epoxy is very hard to beat.

If you apply any coating without prepping the surface as described above you are just trapping contaminants against the aluminum and the aluminum will continue to corrode under the coating. Do a search for poultice corrosion for details.