Reinforcing Design-Cast 66 with Fiberglass

In order to obtain the best properties, you should reinforce DESIGN-CAST by the addition of fiberglass—especially when you are forming thin sections. Fiberglass provides high flexural strength, and high resistance to impact or chipping.

Types of Fiberglass to use

Different forms of fiberglass are useful for different purposes:

• Chopped Strand: Strand length should be one-half (1/2) inch or longer. Use on complex or sharply curved surfaces.
• Continuous-filament reinforcing mat (CFRM): Typically three-quarters (3/4) ounce per square foot in weight. Use on flat or gently curved surfaces.
• Fiberglass Mesh: Typical mesh opening size is one-eighth (1/8) to one-quarter (1/4) inch. Use directly over an armature.

AVOID USING MAT MADE OF CHOPPED STRAND, OR TIGHTLY WOVEN FIBERGLASS FABRICS, since these are not readily wet out by DESIGN-CAST.

Suitable fiberglass is available from the suppliers listed on our Sources page.

How much Fiberglass to use

Use 5% to 10% by weight, based on dry mix weight.

For mat or mesh, this means that you will typically need about four layers for a normal thickness of three-eighths (3/8) inch. In general the more fiberglass, the better—up to a point. IF TOO MUCH FIBERGLASS IS USED, AIR WILL BE ENTRAINED, WHICH WILL TEND TO WEAKEN THE MATERIAL.

Application of Fiberglass for Direct Modeling

Following the mixing procedure given earlier, first mix a batch of DESIGN-CAST 66 small enough to be used up within its working time.

When using chopped strand fiberglass, mix the proper amount of the fiberglass into the mixed batch of DC-66 by hand, using a stick or spatula. Apply the fibered mixture onto the armature, and form into shape. Continue applying fibered layers of mix until you reach the desired thickness and shape. You can then obtain the final texture and detail by applying a top layer of mix without any fiber in it. The top layer should typically not be thicker than about one-sixteenth (1/16) inch.

When using fiberglass mat or mesh, first cut the mat or mesh into manageable size pieces. One useful technique is to first saturate pieces of the mat or mesh with freshly mixed DC-66 paste, using a brush, spatula, or squeegee, working over a flat plastic-covered surface. Then lift up the mat or mesh and apply it to the armature. Continue to build up successive layers until you reach the thickness you want. Whenever possible, overlap any joints between adjacent pieces of fiberglass, with places of fiberglass in the next layer. When you reach the desired thickness, apply the final unfibered coat.

An alternate technique is to brush or trowel a layer of DC-66 mix onto the surface of the armature. Then press pieces of mat or mesh onto the paste, using a stiff brush or other tool to help saturate the mat or mesh with the mix. Continue applying alternate layers of mix and mat or mesh, until you reach the thickness you want. Apply an unfibered layer last.

Make sure you keep the piece uniformly moist during modeling. Finally, proceed with curing and finishing.