Originally posted by Creative Chicks:
...Just keep in mind gator foam or mighty core, which ever you go with is not acid free, so you need to line it with an archival material...
In framing we have two gas-impermeable barriers that are practical for most applications: glass and metal. Unfortunately, neither of them is practical for this job.
If one or two layers of 4-ply alphacellulose board are used, that would stop direct chemical migration for a decade or two at least, but would not stop gaseous chemical migration, which could come from the expanded polystyrene core of Gatorfoam. Especially in a closed-up frame, that could be a bigger problem.
How much preservation do you want? If 10-20 years is enough, with the possible problem of off-gassing, then Gatorfoam/MightyCore would be an OK substrate. But staples will not hold in its edges, so you might want to lace the needlepoint.
If you must use a board backer, fluted polypropylene would be much better. You should still line it with alphacellulose, for a smooth surface and to pad the edges, but it would not cause any chemical problems over time. You would have to lace it, as well.
Another possible mounting board would be 1/4" acrylic. With sanded edges, it would be among your best choices for preservation purposes.