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.