DuPont makes Variprime 615S/625S Self-Etching Primer. It has been
recommended to me by a retired FAA engineer.

As far as I know, this self etching primer needs to be coated with a sealer,
such as urethane paint. O It is porous, and therefore is not a moisture
barrier.

I use Akzo Nobel 2 part epoxy primer (the ugly lime green stuff). You need
to do the alodine thing (pain in the butt), however the surface now is
scratch proof and moisture resistant. It meets Boeing spec. BMS 10-11 for
fuel resistance etc. Very good stuff, and about the same cost as Variprime
if you include the Alodine. Labor cost is the only variant. You can get
other brands that meet BMS 10-11 as well.

You can also get some self etching spray bombs, such as Marhyde, however
they scratch very easily.