Back of the wings, use waxoil, the underseal... better than factory.
Personal opinion, but I'd stay away from that stuff. In fact any underseal is crap IMO.
Whilst it remains flexible it also stays sticky. That means all the road dirt will stick to it too. As you probably also know, salt and mud covering the underseal will eventually find its way through and corrode the panel behind it without you knowing. Bastard stuff that rust is!
If it were me I'd spray the back of the new wings prior to fitting. A decent zinc rich primer such as Bilt Hamber's Electrox. Then I'd ask at your bodyshop what they use on modern cars for underbody protection, you know - the flexible mottled finish paint similar to stonechip. That way, if you want you could finish the underside of the wings in the same colour as the outer panels or just matt black for the O.E. look.
Then fit the wings using seam sealer between the joint faces.
Job's a good'un and no nasty sticky underseal.