If you can salvage some of the aluminum siding by cleaning away the corrosion on the underside, it can be treated with a paint primer containing zinc chromate that is made for aluminum. Also there is a product called alodine that is used in the aircraft mfg industry that is used on bare aluminum to prevent corrosion. That is what I did when replacing some bottom floor wood with treated marine plywood at the contact points. I had the local napa order the zinc base primer for me. I believe the alodine can be purchased on line. Maybe someone with more experience working with aluminum can chime in.
