How to Make Your Own Polyurethane Molds for Making Stone


Creating your own fascia stones out of concrete, using polyurethane molds you make yourself, can save you time and money. An important part of the faux-stone casting process is curing; the concrete stones must cure for several days before they reach full strength. So the more molds you have, the more stones you'll get out of a single curing cycle. Plus, with more custom molds, you can cast a greater variety of stones to give your final project the look of real concrete masonry.

Things You'll Need

  • One or more open-topped wooden boxes, about twice as deep as the stones you want to cast
  • Flat-bottomed fascia stones for models
  • Spray acrylic sealer
  • Spray mold release
  • Two-part urethane casting compound
  • Measuring buckets
  • Large mixing bucket
  • Hand mixer
  • Arrange the model stones in the bottom of the box, flat-side down. The stones should be about an inch from each other and the sides of the box.

  • Spray the surfaces of the stones and the exposed sides of the box with spray sealer, and allow to dry per the manufacturer's instructions. Then spray the stones and walls with mold release.

  • Measure out the two parts of the urethane mold-making compound, using the proportions specified by the manufacturer. Combine the two parts in a large mixing bucket with an electric hand mixer. Then pour the urethane over the stones in the box. The stones should be covered to a depth of at least one inch.

  • Allow the urethane to completely cure, per manufacturer's instructions; this typically is least four hours. Invert the box and allow the urethane mold to drop out. Remove the stones and clean debris from the inside of the mold.

Tips & Warnings

  • It's best to use other faux stones as models because they have perfectly flat bases that won't let liquid urethane flow underneath them. If your stones don't lay perfectly flat, seal the bottom edges with silicone caulk and allow the caulk to set up before pouring the urethane.
  • Two-part urethane has a limited work life before it begins to set up. Make sure your stones are completely positioned, sealed and coated with mold release before you begin to mix the urethane.
  • Two-part urethane emits harmful vapors as it cures. Allow the mold to cure outdoors or in an area with adequate ventilation.

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  • Photo Credit Jupiterimages/ Images
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