RE: using expanding foam in built up wings
I agree with Krener. Don't do it.
I use that expanding foam to fill sheeted retract areas. The way I do it now is to sheet one side, fill with foam, let sit for 24 hours, cut off the excess and sheet the other side.
The REASON I do it that way is because the first time I tried it, both sides were sheeted, the hole was cut for the gear and I added the foam. The next morning, all of the sheeting had busted open from the expansion.
Ribs do very little structurally. If mice have eaten holes in them it should really be no problem. I have often cut out a LOT of the centers of solid ribs for lightening with no ill effects.