Ply Underneath Aluminium Fascias

[oursurveysaid]

I'm about to fit a couple of bespoke aluminium fascias to my extension, replacing the rotten plywood and timber mess left by the builder.

My question is: what should I do to the new plywood I'll be installing, which will then be clad in the aluminium? Can it just be primed or sealed, or does it need several coats of paint as if it were being left exposed? In theory it'll be kept completely dry by the aluminium but I don't want it to rot if the gutter starts leaking or something.

Any suggestions for appropriate products are welcome.

Cheers