I also have an old Genisis foam wing that was sheeted but never used and one day I plan to build an airplane around it. The sheeting on the leading edge has separated and I've spent the last couple years thinking off and on of ways to fix it without destroying the wing.

Think what I'm going to do is load up a hypo with foam friendly CA and just give it shots, hopfully between the sheeting and foam. Even bought the CA a couple of times and ended up using it for other stuff before I was able to try my idea. One thing that has stopped me is I don't have the blanks that came out of the cores to insert back in to be sure I'm not gluing something out of shape.

If anyone comes up with a better plan I'm listening...

Yea, the best bet is to inject epoxy under the covering. If you have cradles hanging around somewhere, You could cut them to fit and once the epoxy is in, set the wing in the cradles and weight it down.

I didn't build it originally, but I cut off all plastic pieces and formed new balsa parts. Lots of sanding, and I ended up re adhering sheating with epoxy. I just cut along the grain, pried up the loose spot and smoshed some epoxy into gap. Then supported wing with foam block, placed a piece of hard wood on top of seam with several weights on top of that.(I Did have waxpaper on wing) Worked great, no lumps or bumps. Thanks to all who responded

I have had to do that on one of my large R/C airplanes. I used a sharpened brass tube to cut out a plug in the loose sheeting. Inserted some poly glue in the void (not too much) and weighted the skin down. After it set, I replaced the plug and finished over it. It looked ok. I suppose if it were real loose or a large area you could slit the skin and rejoin so it would lay flat.