My thoughts as a retired aircraft paint shop owner are this:
The fancy modern polyurethane paints can trap moisture and corrosive salts between the paint and the skin. Urethane is more waterproof than the old enamels. Water will get in, but the moisture cannot get out.
The only fix is a layer of primer. Alodine without primer might work but alodine with epoxy primer is best to protect the aluminum from the topcoat.