I use ProBond for almost everything.

-It adds time to building but pays for itself in repairs down the road.
-It ends up weighting less than epoxy for gaps and corners.
-And since it is a polyurethane it resists fuel.

I have seen a test on a broken wing that had the foam rotted out and left the penetration of ProBond. It was amazing!! This stuff really gets in there.

Check this link for more interesting info on choices.
http://www.scaleaero.com/probondwingskin.htm