Epoxy will work fine, and need not be heavy at all. A PAIR of 78" bipe wings can be done with < 4 oz per wing, if you do it right. This method will still work for your 'eaten" cores, if the eaten areas aren't too extensive, and aren't on the leading or trailing edges.
You WILL absolutely need 3 (maybe 4) things for this to work..if you don't have them, stop reading now.
a) The wing shucks
b) A FLAT table. no bumps, waves, etc. Level is not necessary, but FLAT is.
c) Preferably a vacuum bagging system...if not, LOTS of weight (concrete blocks, car batteries, etc) is a suitable alternative. If you use heavy weights, you'll also need a large FLAT board...like a sheet of 3/4" MDF or something...that's at LEAST the size of a wing.
1) Get some poster board. Lay it in a shuck, put the wing core down on top of it, and put the other shuck on. Trace around the wing, leaving a good 1" or so overlap on all edges. You don't need to do 4 patterns...2 will do...a top and bottom. They can simply be flipped for the other wing.
2) Cut your 'templates" out of the poster board. Use these to plan the various holes, bays, etc you may need for servos, or retracts, or whatever. Cut them out.
3) Lay down sheets of balsa, and tape them together (not glue...tape) on what will be the top or exposed side of the sheet. Cut them to match your templates, including any of the holes for servos and such.
4) Lay in any spars/false ribs/servo bays/etc that you may need to add, prior to sheeting the wings.
5) Spray a bit of hair spray on the "foam side" of the sheets. yes. hair spray. plain ole hair spray. Don't soak them...just get them kinda tacky.
6) Now..prepare to sheet the wings by first laying a large sheet of plastic film/saran wrap/etc over your build table. This will keep drops of epoxy from getting on the table, making it "un-flat".
The next steps need to be done quickly and in order...so have everything prepared first.
7) Mix up a batch of 30-60 min epoxy. 60 min if you think you'll want the longer work time, but 30 will hold fine. For that bird, you'll probably NEED < 2 oz per TOTAL wing...for ease of working, mix up 4-5 oz per wing, and just accept the waste the first few times you do this, until you get a feel for how much a given wing will need.
8) Lay the sheet being epoxied down on the plastic film, tape side down. Pour some epoxy on there, then...using a credit card, or one of those "window tint" squeegees, or something like that, spread that stuff as THIN as you possibly can. Quite literally, if you're doing it right, the balsa won't even look wet. It'll BARELY even be sticky. I can ABSOLUTELY promise you that you will SWEAR there's not enough epoxy on there. There is. If you cover the whole sheet, there's enough.
9) Squeegee some more, because you didn't believe step 8. Seriously...the balsa will NOT look wet. It'll be just SLIGHTLY sticky. Really scrape that stuff off there, and spread it just as THIN as you POSSIBLY can.
10) Do the other side, if you wish to sheet both top and bottom of a wing at the same time. if not, that's fine. First couple i did, i did one side at a time, just to not be rushed with an unfamiliar process.
11) Do the other wing, if you're doing both at once. See comments in #10.
12) Make a "sandwich" that goes Bottom Shuck, Sheet (epoxy up), Wing core, Sheet (epoxy down), Top shuck. Be careful to line the wing core up very precisely in the shucks.
13) Put the wings into the vacuum bag, or set them on the flat table, set the large flat board on them, and weight them down...a lot. I normally put 40-60 pounds on each wing. the top board allows the weight to be spread out evenly, and push down uniformly on the entire wing...the same effect created by vacuum bags.
14) Clean up your mess
15) Go away, and come back tomorrow.
16) Pull the wings out, trim the overhanging sheeting, and finish as desired.
17) Enjoy your INCREDIBLY light, AMAZINGLY strong wings.
==================
Couple of random comments here now.
This method was taught to me by a guy who, in his day, built wings for some awfully well known champions and pros. It's not some random made up way of doing things...it's a proven method, over several decades, on some of the highest profile airplanes in the hands of some of the most demanding competitors in the hobby.
While not necessary for wings the size you're dealing with, on larger (1/4 scale, 2m pattern planes, etc) wings, a layer of carbon fiber veil can be used in between sheets and core. In that case, epoxy the CF only, not the sheets. VERY strong sandwich.
Despite steps 8 and 9 above, you're going to use more epoxy than you need.
That's fine..everyone who's ever tried this method did so the first few times. The key thing to understand is that there really only needs to be enough to contact wood and foam. Any more is simply wasted...it's dead weight. Truly excessive amounts can create bumps/ridges in the wing surface. As examples:
33% Pitts S2S with false ribs used 7.5 oz...both wings. 78" wingspan
25% Lanier Cap232 kit. We only MIXED 5oz total for both wings...and there was waste.
.40 sized Pitts S2S, fully sheeted wings. 4 oz was GOBS too much for both wings.
The most important "trick" is to make SURE the entire surface of the sheeting is covered...ESPECIALLY the edges. You get it all covered, they'll be strong enough to hit golf balls with (literally, I've seen this done).
=========
The process sounds more complicated than it is...just follow the steps, and it'll be simpler than you realize...and you'll end up with some very nice...and truly light....wings.