What is the best sealer for diamond art?

A clear sealer made for diamond painting, in a gloss finish so the drills keep their sparkle. It comes as a liquid you apply with a brush or sponge, or as a spray, which is quicker but covers less evenly.

Roll the canvas first to press the drills in, test the sealer in a corner, then apply one thin coat and let it dry flat. If the picture goes behind glass, you do not need a sealer.