Sorry to be late to this party; had a bunch of things to do to get ready for my club's Electric Fun Fly this weekend.
I've covered a few foam wings in my life, and I've come up with a system that works for me. I did a tutorial several years ago on the subject and have discovered that someone posted it on the Internet without consulting me. That's okay, I'm getting used to that sort of thing...
Anyway, here's the link to that tutorial. One caveat: The photos that show the Z-Poxy being spread on the balsa shows a lot more epoxy than is actually used. I had trouble with the photo showing the more minimal amount; it was almost imperceptible. So, I left more on just to clarify the spreading with the notched piece of plywood. As Brett has pointed out above, only a very small amount of epoxy need be used. And, as he pointed out, a lot of weight is the other key. I use upwards of 300 pounds of weight on the "sandwich."
That is a very critical point - you want to spread around a fair bit of epoxy, then remove almost all of it by scraping it with a notched scraper (Signature Inn key card, etc). There is more-or-less no standing puddles or continuous lines of glue, it just looks like you got it damp. The total weight for a single wing panel (top/bottom) is less than 1/2 ounce. So you mix up way more than you need, pour it on there, spread it around, and then scrape it almost all off.
Another point - watch out for it curing too quickly, particularly if you mix it in a cup. You have to have it remain liquid the entire time, and it takes a while to do the spreading and setting it all up in the cradles. The problem with mixing it in a cup is that it self-heats, and heating it speeds up the curing process. Hobby Poxy Formula II was notorious for this - mix it in a 1-ounce cup, start applying it, and then next time you reach for the cup for more, it is warm to hot. Leave it alone, and it gets hot enough to *boil* in high ambient temps. Instead of your "30-minute" working time, it might start getting stringy in a few minutes.
A trick to avoid this is to get a cookie sheet or something similar, cover it with smooth aluminum foil, mix it up, then pour it out in a puddle on the foil. That will let it spread out and keep the working time as intended. 30 minutes is *marginal* to do this, I usually only do one panel at a time.
How bad the self-heating might be depends on the brand and age of the glue. EZ-Lam in particular is extremely good about self-heating, it barely gets warm, and the working time of 30 or 60 minutes is exellent, it really is still a liquid at the end of the time. Various West systems I have tried is OK. A lot of the "finishing epoxy" types are really bad, and will "go off" in ten minutes and you will not be able to pick up the cup it is so hot.
I also note that these are *very powerful chemicals" and you should avoid exposure or getting it on your skin if practical. Small amounts are not the end of the world but take some precautions and don't get is smeared around all over you hands, etc.
Brett