Things You'll Need
Sheet of balsa wood
Spray adhesive or epoxy
Books or other heavy objects
10-18 gauge wire, straight
Foamcore is a useful medium for many crafts, but can present a problem when in contact with moisture like paints, dyes, and condensation. Foamcore is simply a sheet of fine-grade foam sandwiched between two sheets of thick paper, and moisture causes curling by weakening the paper layer and popping the air bubbles in the foam, causing distortion. The best way to keep foamcore from curling is to frame it, but if this is not practical, there are a couple of alternative methods.
Cut a sheet of balsa wood to the same dimensions as the foamcore. Absolute precision is unnecessary, as edges will be evened up later.
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Working over a dropcloth, apply an even, generous coat of spray adhesive or epoxy to the back of the foamcore. Work from the center to the edges.
Place the foamcore on top of the balsa, glue-side down, lining up the edges. If any glue comes out from between the pieces, wipe it away with a damp cloth.
Evenly weight the foamcore with books or other heavy objects to ensure a secure press. The weights must be evenly weighted, or the foamcore will adhere unevenly, which can actually cause curling.
Let dry undisturbed for 24 hours. This ensures a tight, even bond.
Lightly sand the edges of both the foamcore and the balsa to even out mistakes and create a uniform edge. If the balsa wood sticks out much further than the edge of the foamcore, a razor knife can be used to trim it more quickly than sanding.
Using wire cutters, cut several pieces of wire to the length of the foamcore. Exact measurement is unnecessary, since ends can be trimmed later.
Thread the wires vertically through the center of the foam about one inch apart down the length of the foamcore piece. Heating the tip of the wire with a lighter can make this easier.
Cut several pieces of wire to the width of the foamcore. Again, exact measurement is unnecessary.
Thread each wire horizontally through the center of the foam, about one inch apart, across the width of the foamcore piece. There should now be a wire grid in the center of the foamcore.
Using wire cutters, trim any excess ends that may be showing. The best way to do this is to pull the wire out a tiny bit, snip off the end, and push the wire back in so that the cut edge is actually just inside the foamcore instead of even with the edge.
Mounting is better for pieces that do not have much weight on the front side, like a mounted poster or a painting. Paper and paint do not add much weight to the foamcore, so the balsa wood is a more than adequate support. Wire bracing is better for pieces that are very front-heavy, such as three-dimensional collages and mounted puzzles. Creating an internal wire grid not only prevents curling, but also adds stability to keep the foamcore from bending or breaking under the weight of the piece.
If a large-enough sheet of balsa cannot be found, the balsa can be mounted around the edges of the foamcore, leaving the center open.
Use spray adhesive for lighter pieces, epoxy for heavier ones.
Do not use any wire thinner than a wire coat hanger, because it will not be strong enough to hold the foamcore flat.