If you don't get responses here, there's a theatre/stage design Google group I've found helpful (sorry, just Google it; I don't have a link for it). Local hardware store might know.
My uninformed guess would be a coat of acrylic medium or very thinned white glue. Is it real Styrofoam--made of tiny pellets--or floral foam, which disintegrates into dust?
Maggie
On 4/29/2005 5:33:06 PM, TeacherArtExchange Discussion Group (teacherartexchange@lists.pub.getty.edu) wrote:
> The drama teacher at my school has created some large
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> styrofoam "rocks" for a school play. After carving
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> and painting them, they are now kind of chipping and
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> pieces are falling apart. Does anyone know of a
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> particular sealant or spray that he can purchase to
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> seal the foam pieces? Marcia L in Illinois
>