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Casting Polyester and Building Fiberglass Parts

In selecting a moldmaking rubber for casting polyester or building fiberglass parts, you must be certain that the systems are compatible. Silicone molds are a great choice since they are flexible and offer excellent release properties.
There are two types of silicone rubber and two types of polyester resin.

Platinum-catalyzed silicones are your best choice since they are compatible with both types of polyester. Whether you want to cast clear polyester parts, cold-cast marble, or build a fiberglass part (boat, paneling, disk, auto hood, spolier, etc), the best choice is one of the Platinum-cure silicones. However, platinum-cure silicones require additional precautions when pouring over original patterns because it's cure can be inhibited by certain surfaces (wood, adhesives, tin, some types of clay). To prevent this inhibition (which causes a sticky surface), all areas of the original and mold box must be sealed.

Tin-cure silicones, on the other hand, will only cure one type of polyester (other types of polyester will not cure along the interface - they will have a sticky surface).