I use Mod Podge to seal the glass-framed Diamond Paintings that I make. It's a decoupage medium (a glue and sealant in one) that comes in both matte and glossy finishes. You can find it at most craft stores. First, brush a thin layer of Mod Podge over the entire diamond painting surface. Be sure to get into all the nooks and crannies between the diamonds. Let it dry completely (usually takes about 15 minutes). Then, apply a second coat of Mod Podge, again making sure to coat all the surfaces evenly.