Mitch,

whether using epoxy or PU, the technique is more or less the same. The idea is to apply abundant amounts of pressure to the stack while curing. I know some love contact cement (Sorghum) but I find it is harder to control the amount of glue due to its viscosity. The cement also is a "4 layer" bond method since it is first layered on to each surface, allowed to dry (thereby bonding to that surface), and then the two cement surfaces are placed into contact (hence the name) and the bonds allowed to "interlink" with each other. I find that this makes for a heavier system.

The epoxy/PU approach is different. While PU has more penetrating power (unlike contact or epoxy), I'm not sure that this is needed (although I'm sure David's wings are rock solid). Not that there is anything wrong with using it. My understanding is that the idea is to prevent the skins from shifting laterally on the cores. This shear resistance is what allows the wing to withstand longitudinal stress (as when coming out of a loop). Things like CF spars help the wing to withstand torsional stress (as in a snap). So with that in mind, the key is to insure that the skins adhere to the cores within a depth of the sheeting thickness (i.e., 1/8" for most 1/16" skinning). Epoxy will do that.

Laminating epoxy (Zap's is great, GP's not so much) flows real nice. It is not gooey at first. You only apply it to your trimmed skins with a brush or roller (I find the brush absorbs less glue and it is expensive) and then once the entire surface is painted you take a CC or Bondo scraper and pass over the skins removing the glue into the container until they appear "wet". The should have a uniform sheen to them. If they appear glossy, you have too much glue. Make sure there are no dry spots.

You then stack your sandwich using wax paper between the shucks and the skins if needed (depends on how clean you were with the glue). Try to remove any epoxy from the external surfaces using alcohol on a finger - once it's dry, it's hard to remove as we know. I prefer to lay out the two wing cores side by side rather than one atop the other. This ensures that if for any reason, the table has a 1/16" (or less) warp to it, hopefully the warp will be symmetric about the panels. If they are stacked, you'll introduce identical left or right wing warps so the wing will not be uniform once you flip one of the panels. Lay them out TE to TE with the roots on the same side of the table.

You can hold the top shuck on with a battery or other weight while you prep the "mother load". Ideally you'd then use something like particle board or better yet, veneered particle board (it's flatter). Make sure your table is strong. Lay the board atop the two cores making sure the shucks haven't slipped - very important. Then find some 100 lbs of "stuff" (the heavier the stuff the better) and distribute it evenly over the top board. I don't think you can add too much weight. It's amazing what foam can withstand! The thing to watch out for are the joints at the LE and TE. The shucks ideally would "curl" around these areas but unfortunately they don't so you have to improvise. Clamps on the LE/TE ends help with this but I tend to be concerned with clamping for reasons of non-uniform pressure on the panels. To me, the more you can spread out the load, the better. You want the same PSI over the entire core.

Leave the mess overnight, remove the load 24 hours later (at least 12) and you'll have very true wings provided your bottom and top surfaces were flat. You can then take a 12" sanding bar and with the wings stacked on top of each other and weighted to symmetry, sand the skins down to the core/shucks. This way you also ensure identical panels.

Perhaps a long winded break down but that's the idea. Give it a try, you'll like it. It's probably a little more time consuming than cement but I think the results are worth it. I suspect your wings will also be lighter.

David.