What would you be using it for on your JetPack? I don't know what you're doing but I can't think of any other use other than thinning it down and spraying it as a sealer for your paint (Matte version). Then again, there many other sealers that are more durable.
I am making the pack from the foam board that walmart sells. I'll need to fill the seams when I fold the pieces over. I just didn't want to use bondo, and thought I could also give it a better surface for paint
Modge Podge is good for coating like Dahone said. The more "tooth" something has, the better it does. It is basically like a white PVA glue. If you get the foamboard with the smoother surface, I would test build a small piece first and try it on that.
modge podge isn't a filler... it's not really going to hide seams unless you use a LOT of it. i use it sometimes to protect foam from bondo, but i can't see it being useful as a seam filling medium.
Use wood filler, its like a thick paste that dries quickly. it also has no smell and is only about $5 for a pretty good amount. It comes out strong, but easy to sand.
I made the whole pack from the foam board, do I need to spray some kind of sealer on the entire thing after I fix the seams ? Or, do I just paint it as is? Thanks for the wood filler tip BTW, I never would have thought of that.