Things You'll Need
Silicone spray or other mold release agent
Plaster of Paris
Cheesecloth or gauze
Small artist's paintbrush
Natural latex molds, when properly cared for and handled, are quite durable. Sometimes they do tear however, and repairs are necessary. If the mold is in otherwise good shape and the damage is not too extensive, repairs can be simple and straightforward. Small tears may need very little work, but larger tears requiring repair to the mold as well as rebuilding of the plaster mother mold (backing) are still possible.
Remove the rubber mold from its back up mold. Clean it thoroughly—including the torn edges--wash with warm, soapy water; rinse. Wipe it with alcohol to remove any soap residue or oils; let it dry.
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Use super glue to tack the edges of the tear together temporarily in their correct alignment. Use sparingly to prevent buildup that may affect the mold—both on the mold surface and the back of the mold.
Return the mold to its supporting mother mold and press it into place until it keys correctly. Lightly spray with silicone or other mold release.
Mix a batch of plaster of Paris according to instructions on the package. Pour it into the mold and allow the plaster to harden.
Turn the entire mold (with the mother mold) upside down on a flat work surface. Carefully remove the mother mold without disturbing the rubber inner mold or the plaster cast inside it. Use alcohol on a soft cloth to wipe the area of the repair. (The tear should still be visible, but not open--the temporary glue and plaster cast should hold it together until the permanent repair is made.)
Cut several small strips of clean cheesecloth or gauze bandage and set them aside temporarily.
Use a small, soft bristle brush, to paint a thin layer of liquid latex directly on the torn section and the surrounding area about 1 inch beyond the tear all the way around. Position a strip of cheesecloth on top of the latex and press and smooth it until it is firmly in place with no wrinkles. Add strips adjacent to that one to cover the whole area of fresh latex.
Brush a layer of latex over the cheesecloth, saturating the fibers and being careful not to move it. Allow the latex to cure completely. Repeat the application of latex (curing between each application) until a sufficiently strong patch is developed over the tear—usually three or four thin applications will be enough.
Remove the plaster cast from the rubber mold (save for use in future repairs). Seat the rubber mold back in the mother mold and check for distortion under the patched area. If necessary, scrape away some plaster from the inside of the mother mold, a little at a time, testing the fit each time until the mold is supported without distortion.
Seat the rubber mold in the adjusted mother mold, and key it in place. Use an eyedropper to drip a small amount of acetone into the crevice where the original tear was temporarily tacked with super glue—this will dissolve the glue and open up the tear from the front side. Use a cotton swab dipped in more acetone to gently wash the inside of the tear. Let the acetone dry.
Use a small artist's paintbrush to carefully drip a fine bead of latex inside the crevice of the tear. Let the rubber partially cure until slightly sticky. Press the edges together, wiping away any excess latex that squeezes out. Allow the rubber to cure completely before using the mold.
Latex will quickly set on brushes and other implements if not removed. Clean brushes with distilled water between applications. DO NOT use tap water—rubber reacts with chemicals and minerals in the water to bead up and form tiny rubber balls.
Always use solvents like alcohol and acetone in a well-ventilated area. Avoid open flames (and lighted cigarettes!) as well.