The "problem" with automotive primers is that they
contain salts and other minerals that attract moisture.
Primer, in itself, is not weather or UV tolerant and was
not meant to be. It is designed to provide a bond between
the metal and topcoat.
here lies the rub: you have probably seen people riding
around in their cars or trucks that they did some body work
to and sprayed a lot of gray primer on the work and you can
see rust seeping from every pore of the primer that only covers
the metal. Some believe that it also enables moisture to get into
the Bondo which will rust from the bottom out . . . .
So, if you are doing automotive body work, apply a sealer
over the primer if you can not apply the top coat within a few days.
(it says "Primer Sealer" right on the can).
okay - now we are talking about aluminum. It does not rust.
so, that eliminates that worry of the equation. It is very doubtful
that any significant amount of corrosion will form under the primer
or paint any time soon. and it definitely will cause no harm.
My very strong suggestion is to NOT apply a clear coat (of any kind).
it is supposed to be rustic, RIGHT ??? well, faded paint is rustic.
just use it as is - enjoy - go kill some birds.
jus my Dos Centavos
All about Primers = https://www.tinboats.net/primer-and-paint-basics/
.