wood preservative that won't affect upvc fascia cladding?
 

I'm about to clad the fascia boards on my house with UPVC fascia cladding
(9mm thick). The existing wooden fascia boards have a little wet rot in a
couple of places. So I'm thinking of treating those sections with some kind
of wood presevative prior to nailing the cladding in place. I just want to
make sure I don't use a preservative that's going to affect the UPVC in any
way. I think that certain solvents and/or oils might turn upvc brittle. Any
suggestions?

I can't afford a scaffolding and am doing the job with a ladder only, so
replacing the affected boards is not possible.

Is it even necessary to treat the affected wooden fascias, considering that
wet rot stops as soon as soon as the wood is no longer subject to dampness?

Al