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. When completely dry, you can apply an acrylic varnish to preserve and protect your work. Sealing your work, however, will not waterproof it. In fact, exposing it to water will have a negative effect. While a varnish may help protect your creation, please test the application first, as Crayola does not manufacture sealants.
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 .