Do not use spray glues. They are not in any way archival and will stain the paper.
What size are they? You say they are heavily gessoed... how heavy are the pieces themselves?
My first thought would be to float the paintings by hinging the pieces to Cor-X board. Cor-X board is the fluted plastic board used for temporary outdoor signage and is available at any sign shop or in small sizes (20X30ish) at most hardware stores of Michael's, Hobby Lobby or the likes.
Here is an article about how to hinge:
http://frametek.com/HTML/Articles/Float_Mounting.html
It looks more difficult than it really is but might take a bit of practice. If you use wheat paste they would be removable.
The reason I suggest Cor-X board is that it is inexpensive, somewhat archival and readily available. Also if you run the flutes vertically it should hold the weight of the paintings without bowing. If the paintings are large and heavy, then consider using two sheets of Cor-X cross fluted for additional strength.
Foam Core will bow unless they are small paintings even during a short exposition.
For hanging, I'd get some of these:
http://www.govart.com/hardware_hangers_foam_B.html
Any mounting of the paper would be extremely difficult to remove and that is why I'm suggesting float mounting the paintings.
Be aware that since these paintings are on paper they would eventually require glass or plexi over them with a frame for long term display.
Linear Mode
