Both Model Magic and Air-Dry Clay can be sealed once your piece is completely dry. Both products generally take 24 hours to dry to the touch; 72 hours to dry completely. Once dry, an acrylic varnish can be used. We recommend testing its compatibility before applying it to a finished piece. Sealing your work will not waterproof it. Exposure to water and outdoor elements will have a negative effect on the clay/modeling compound, so we recommend displaying your sculpture indoors.
More information about Model Magic is available online at http://www.crayola.com/things-to-do/how-to-landing/model-magic.aspx.
Air-Dry Clay information can be found at http://www.crayola.com/things-to-do/how-to-landing/airdry-clay.aspx .