After joining the wing halves of my foam GWS Beaver I am going to have very significant and weak gaps in the center and I am wondering before I attempt it . . .

Can 30-minute epoxy mixed with micro-balloons be used for filling gaps in foam?

I know it wont sand easy without sanding away the flimsy foam, but its only a cheap $40 foam kit that we are going to toss around in a local park. Although I do want to keep it as clean-looking as possible appearances aren't all that important so it won't matter too much if the balloons isn't perfectly flush with the wing. Just need to know if there will be a problem with it sticking to the foam or if it would melt the foam.

Epoxy won't hurt the foam but I dont think what you're thinking of is acutally going to strengthen anything.

Get the lightest fiberglass cloth you can find and wrap the wing joint with one layer about an inch wide. Use an old credit card to squeege as much epoxy as possible out of the cloth and use 30 minute epoxy so it doesn't set up before you can get it all done. You'll get a very strong joint that doesnt add too much weight if you use the minimum glue possible.

Hi!
Why not sanding the surface first???

your thinking too hard. either sand the two wings so they fit perfect (not nessiary) or just glue them together
this isnt machine class where everything has to fit perfectly, think kindergarden glueing session unless you plan on crazy overpowering it you will never have that glue joint fail. the expoxy is so strong compared to the foam around it the joint failing is the least of your worries, even if its only half glued

my point is dont worry about it, just glue the dang thing together