How to Make Rubber Molds on Fiberglass

You can use a paintbrush to brush silicone rubber in place.

Latex rubber cures through the evaporation of ammonia. Because of this, it must be molded in thin layers on a porous material, such as plaster. Fiberglass is by its nature a very solid material, and as such, is unsuitable for molding in latex. Silicone rubber, however, cures through a chemical process and does not require the presence of air. This makes it an ideal material to make molds of fiberglass objects.

Things You'll Need

• Silicone rubber and catalyst, 30-minute cure
• Mixing bucket
• Scale
• Mixing sticks
• Paintbrush
• Vinyl gloves
• Thickening agent
• Spatula
• Vacuum degasser
• Plaster bandages (optional)

Instructions

1. Combine a batch of silicone rubber with its catalyst in a mixing bowl. The mixing ratio is typically either 1:1 or 10:1 by weight, depending on the product that you are using.

2. Mix the silicone rubber and catalyst thoroughly. Many silicone rubbers are white with a deeply pigmented catalyst so that you can visually see how well it is mixed. There should be no streaks of color. Scrape the sides and bottom of the bucket with the mixing stick many times during this process.

3. Put the bucket of silicone into a vacuum degassing chamber if you have access to one. This will remove all of the air from the silicone, greatly reducing the risk of air bubbles being trapped against the fiberglass, which will cause imperfections in the finished mold.

4. Brush the silicone onto the surface of the fiberglass. This is the detail coat, so really work it into the nooks and crannies. It will want to drip and sag, but you must keep brushing it back in place until it thickens. The ideal depth for this layer is 1/8 to 1/4 inch.

5. Allow the silicone to fully cure until it is no longer sticky to the touch.

6. Mix a second batch of silicone, following the instructions for the first. Add a thickening agent once this batch is mixed. Two common choices are liquid thixotropic agents (which are added by weight) and powdered Cab-O-Sil (which is added gradually until the desired thickness is achieved). You want this batch to be as thick as peanut butter. It will be too thick for a vacuum chamber.

7. Spread the thickened silicone onto the detail coat with a spatula. Be careful not to trap any air bubbles, and allow it to fully cure once it is in place.

8. Add a support shell of several layers of plaster bandages if the mold is still thin enough to be flexible. The rigid shell will help the silicone to keep its shape. If the silicone is thick enough, this step will not be needed. "Thick enough" varies depending on the size of the object being molded, so you will have to use your best judgment.

Tips & Warnings

• Many silicones take hours to cure. If you cannot find silicone that cures in 30 minutes, you can speed the cure time with fast-acting catalyst.

• Always wear vinyl (not latex) gloves when working with silicone.

References

• Smooth-On: How to Make Molds and Castings