Reinforcing Design-Cast 62 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 ½inch or longer. Use on complex or sharply curved surfaces.
• Continuous-Filament Reinforcing Mat (CFRM): typically ¾ounce per square foot in weight. Use on flat or gently curved surfaces.
• Fiberglass mesh: typical mesh opening size is ¹/₈ to ¼inch. Use directly over an armature.

Note: 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 about four layers are typically needed for a normal thickness of ³/₈inch. In general, the more fiberglass the better—up to a point. If too much fiberglass is used, air will be entrained, which may tend to weaken the material.

Addition of Fiberglass: Lay-up Procedure

Following the mixing procedure described above, mix up a batch of DESIGN-CAST 62—small enough to be used up within its working time of 20 to 40 minutes. First brush mix onto the mold surface to give a uniform coating—just thick enough to cover the surface of the mold uniformly—typically about ¹/₁₆inch thick.

Then lay in the first layer of fiberglass. If using mat, push the fiberglass against the DESIGN-CAST coating; then apply more DESIGN-CAST to the exposed fiberglass surface, and brush it in uniformly. When necessary, pre-cut the mat into pieces just small enough so that the mat does not ‘pucker’ when shaped to the mold surface, and lies flat without trapping any air. Whenever possible overlap any joints between adjacent pieces of mat, with pieces of mat in the next layer.

When using chopped strand rather than mat, sprinkle the glass heavily onto the DESIGN-CAST coating, and dab with a brush it to wet in.

If the first layer of fiberglass is applied immediately after the first layer of DESIGN-CAST mix, the glass texture will usually become visible on the face surface of the casting after de-molding. If any such read-through will be objectionable, you must wait until the first layer of mix has thickened before laying in the fiberglass (usually about 20 minutes) and then you apply the first layer of glass. Continue to apply alternate layers of DESIGN-CAST and fiberglass in this way, until you obtain the desired thickness of casting. After the first layer, you don't have to wait between successive layers. The normal casting thickness of fiberglass reinforced DESIGN-CAST for general purposes is ³/₈ to ½inch.

The working time of mixed DC-62 is normally 20 to 40 minutes, depending on temperature, humidity, and the amount of water in the mix. In dry or warm weather you should keep the working material damp, using a water mister (such as is used for house plants). In any case, when you leave the work for a while you should spray a water mist over the surface of the work, and/or cover it with a plastic sheet so as to prevent water from drying out prematurely.

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