Guide to Working with Plastics: A Primer on Poured Shapes--Glass Fibers Reinforced with Plastic Resin

Few building materials can match the versatility and utility of home-mixed glass-fiber-reinforced plastic, commonly called fiberglass. Strong, lightweight and completely weatherproof, it can be molded into free-form shapes, such as basins or furniture, or laminated to decorative wall panels, sills or roofing. Formed of woven or felted glass fabric embedded in a plastic resin, the material combines the best attributes of both. The strength of glass fibers in the fabric rein forces the weak, brittle plastic; the plastic makes the material rigid and gives it a smooth, impermeable surface.

For an even stronger material, the pro portion of glass to resin is increased with in a range of 30 to 70 percent of the total weight. The glass content is determined by the density and arrangement of the glass filaments in the fabric and by the amount of resin used to laminate it.

The fiberglass fabrics used in laminating are all made up of rovings, bunched strands of glass filaments. The rovings are pressed or woven into fabrics of varying densities; density is measured in terms of the weight, in ounces, of 1 square foot of fabric. The fabric weight, as well as the forming method, deter mines the quantity of resin needed. Generally, a chopped-strand mat takes double its weight in resin; the ratio of resin to fabric for a woven scrim is one to one.

Fabric choice depends on the amount of reinforcement needed and on the type of resin to be used. The bonding agents in fiberglass fabrics are designed to bond with different resins; in purchasing the fabric, be sure to specify the resin you have selected. Because woven fabrics, which are stronger, don’t bond as well with each other as they do with mat fabrics, it’s best to alternate layers to ensure even strength throughout the laminate. And for a smooth surface, a fine fiber glass mat is often used for the layer just beneath the final coat.

Although many resins are suitable for making fiberglass, polyester is both the simplest to use and the most economical, and it provides excellent strength and moisture resistance. When laminated, polyester’s normal shrinkage of 10 to 15 per cent during curing is reduced to 2 percent or less, and this slight shrinkage is often an advantage: In molded laminations, it allows the finished object to slip easily from the mold. Polyester resin is usually available as a two-component system, with resin in one container and hardener in another. Be sure to follow the manufacturer’s mixing instructions exactly, combining the components carefully to avoid mixing in air bubbles, which weaken the cured resin. For de tailed information on measuring and mixing resins.

Usually the resin includes an accelerator, which speeds the curing time, but sometimes this component must be added separately. If you are adding an accelerator, be absolutely sure to mix it into the resin before introducing the hardener. Mixing the accelerator with the hardener can result in an explosive chemical reaction between the cobalt naphthanate in the accelerator and the peroxide in the hardener.

A special resin called the gel coat is always used as the outermost layer in fiberglass lamination. This unreinforced resin provides a smooth, glossy, protective layer between the glass fiber and outside moisture. It’s applied as the first layer if the fiberglass is being built up in a mold, but as the final layer on a flat lamination. By using a tinted gel coat, either premixed or mixed on the job with up to 10 percent of a suitable polyester tinting paste, you can impart a surface color to the fiberglass.

Gel coats come in several formulations. An air-curing type is easiest to handle; it’s viscous and less likely to run or, when dry, to crack. Ideally, it should be applied with a brush in a single coat. If it’s the first layer, it should be thoroughly cured before the laminating resin is applied; otherwise, the solvent in the laminating resin could react with the gel coat and damage its finish. The gel coat should also be compatible with the laminating resin; information about compatibility is included on the label on the can.

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A Glossary of Glass Fabrics for Laminating

--Chopped-strand mat. The most common fiberglass reinforcement is a felt-like fabric of short, randomly arranged glass strands held together by a binding agent. Standard 1.5-ounce mat is well suited for making reinforced fiberglass roofing sheets and wall panels. Mat fiberglass is stiff and difficult to form until the binding agent dissolves in the resin.

--Woven roving. Excellent for reinforcing walls, joints and corners, woven roving provides great strength. Because the coarse weave tends to cause air bubbles to form in the resin between layers, woven roving is usually alternated with layers of mat to prevent delamination. The coarse weave also makes woven roving unsuitable as a surface layer, since the weave shows through the surface resin. Woven roving in weights greater than 2.18 ounce is too stiff to be used with ease in home workshops.

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When you are laminating fiberglass with a mold or form from which it will later be removed, you will need a release agent to keep the fiberglass from sticking. For simple molds, you can coat the mold surface with lacquer spray or cover it with a sheet of polyethylene film. For large or complex molds, a two-stage parting compound of hard wax and polyvinyl alcohol is generally used; it’s available from plastics suppliers. Apply the wax first and let it dry completely; then apply the alcohol, using a sponge to lay on a thin, even coat. Protect the surface from dust while the parting compound dries.

Cut all the fiberglass fabric to the proper size before mixing the resin. Most such fabrics can be cut with scissors or a utility knife, or they can be torn by pulling them over the edge of a handsaw. If the cut fabric edges are to be lapped, comb or fray the edges of the glass strands to flat ten them so that they will intermingle within the resin and make a joint without a visible seam.

To impregnate the fiberglass with resin, use a paint roller or a brush, stippling the brush over the fabric so as not to dislocate the glass strands with the sticky, resin-coated bristles. Never apply more than 6 ounces of resin over a square foot of fabric; the heat generated by the resin as it cures can adversely affect the laminate if it’s applied in quantities too great.

Some polyester resins cure completely only in the absence of air; left exposed, they remain tacky indefinitely. This can be an advantage in multilayer lamination over a large area, where new coats are usually applied before the previous coat cures—a forming process called wet-on-wet. The work can be interrupted for several hours without harming the laminate. When using such resins, however, the final coat must be sealed with a gel coat or covered with an airtight layer of polyethylene film until the resin cures, after which the film can be peeled off. Or the final coat can be painted with acetone, which dissolves the uncured resin but leaves a rough surface.

Because working with resin is messy, use disposable utensils for measuring and mixing it whenever possible. Keep tools soaking in solvent so that you will be able to clean them when the job is completed, before the resin hardens. A supply of clean rags is essential for removing spilled resin, and the floor beneath the work should be covered with newspapers to catch drips, which are difficult to remove once they have had time to harden. In addition, the laminating process also requires several specialized tools. One is a metal disk roller for consolidating the resin and glass and pushing out air bubbles that form between layers. Rollers come in several forms, with washer- or paddle-shaped blades, and in sizes ranging from 1/4 inch to 12 inches wide. For contoured surfaces there is a flexible roller, with a head resembling a length of coiled spring. To trim and finish the completed lamination, you will need a metal- cutting saw, a forming tool, and a supply of wet-or-dry sandpaper graduating from 240- to 600-grit.

Most of the components used in laminating fiberglass are irritants. Always wear a respirator and goggles when cut ting or sanding fiberglass to protect your self from fine glass fibers released into the air, and protect your skin with when you handle the material. Both the resin and its hardener are caustic, flammable and toxic. Work with them in a well-ventilated place, away from flame, and keep them away from skin and eyes.

Because of the large quantities of resin often involved, laminating is best done outdoors, where the fumes are less dangerous. But don’t work outdoors when temperatures are below 65° F., or the res in will cure too slowly. Also, avoid direct sunlight and hot or cold drafts, which might adversely affect the curing process.

--Glass scrim twill. Finer and more flexible than standard woven roving, scrim twill is a good choice for covering irregular shapes and com pound curves. Standard .55-ounce scrim twill offers excellent resistance to stress, and is often alternated with mat for high-strength laminates.

--Surface mat. A felt-like material, finer than ordinary mat, surface mat ranges in weight from 0.6 ounce to 1.1 ounce. It’s used as a smooth final layer to conceal the texture of coarse fabrics.

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Weatherproofing a Table for Use Outdoors

1. Cutting fabric to fit. Cut .55-ounce scrim twill to cover the top and edge of the table, adding 2 inches along each edge to lap to the under side. If the tabletop is too large to cover with one piece, cut two pieces with a 1/2-inch overlap; then fray the overlap until the strands of fabric can be intermingled and pressed flat. Tack the fabric to the table edge near each corner, then cut away a square of fabric at each corner so that the fabric will fold flat at the corners; again allow for about a ½-inch overlap around each corner, and fray the overlap so that the strands can be pulled around the corner.

Remove the tacks and carefully lift the fabric from the table. Using an orbital or belt sander with coarse abrasive paper, sand the tabletop and edges to remove all paint, dirt and grease; round all the edges. Countersink any screwheads or boltheads, and fill all holes with wood putty.

2. Priming the surface with resin. Using a paint roller, apply a thin primer coat of resin to the top and edges of the table and to 2 inches of the underside. Mix the resin according to the manufacturer’s instructions, then thin it to the consistency of paint by adding a bit of acetone.

Allow the primer coat to harden partially (about 45 minutes) while you clean tools and mixing containers with acetone. When the primer coat starts to harden, mix another batch of resin but don’t thin it. Then, using the roller, apply a second coat over the primer coat.

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3. Laying the fiberglass fabric. Have a helper hold one end of the precut fiberglass fabric away from the resin-coated table while you align the other end with one corner of the table. Then lower the fabric into the resin, lapping it over the edges and underside and intermingling the frayed strands at the corners. Avoid wrinkles; if the fabric must be moved, lift it and realign it. When the fabric is in place, stipple it with a resin- filled soft-bristled brush until it’s completely flat.

For a large table, repeat this process, covering the other half of the table with fiberglass. Overlap the frayed edges of the two pieces of fabric by ½ inch, flattening the frayed strands so that the seam line will be invisible.

4. Rolling down the fabric. Force the fabric into the resin with a disk roller (inset), working from the center of the table out to the edges. Roll out any air bubbles, then check to be sure that all the fabric is saturated with resin. Let the resin cure for two hours; meanwhile, clean the tools.

Cover the laminate with a pre-sealing coat of resin, adding tinting paste if you want color in the final surface. Use a soft-bristled paint brush to lay on the resin in an even coat, about 1 inch thick. When this coat has hardened completely, sand down any uneven spots with 320-grit emery paper.

Mix enough air-drying gel coat to cover the laminate in one thin application. If you require more color, you can add up to 5 per cent tinting paste. Apply the gel coat in the same way as you applied the pre-sealing coat.

Molding a Base to Use in a Shower Stall

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1. Making a pattern. Cut from brown wrapping paper a pattern that will be large enough to cover the mold. Then shape this paper pattern to the mold, cuffing away the excess paper at the corners but allowing for an overlap of ½ inch wherever possible.

2. Applying the parting agent. Use a rag to apply a coat of paste wax on the surface of the laminating mold; polish the wax by rubbing in a circular motion with a lint-free cloth. Apply and polish two more coats of wax. Then use a square- edged plastic sponge to apply a thin, even coat of polyvinyl alcohol over the entire mold. Work with smooth strokes, always in the same direction, and overlap successive strokes to avoid gaps and streaking.

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Use the paper pattern to cut four identically shaped pieces of fiberglass fabric: two of 1.5 ounce mat, one of 0.55-ounce twill, and one of thin surface mat. Cut the fabric with sharp scissors to the exact outline of the pattern. Fray the over lapping edges as in Step 1.

3. Laying on the laminate. Mix a batch of air-drying gel coat sufficient to cover the entire mold. Use a soft-bristled paintbrush to apply the gel coat to the mold and to a small piece of scrap wood. When the gel coat on the scrap wood is tacky (usually in about 30 minutes), mix a batch of laminating resin, enough that it will cover the entire mold.

Using a paint roller with a ½-inch nap, coat the mold with laminating resin. Then lay the pre cut fiberglass surface mat over the mold, using a helper to align it (Step 3). Work the fabric into the resin with a soft-bristled brush and remove air bubbles with a disk roller, taking special care to flatten the loose strands at the corners. Before the resin cures, apply successive coats of resin and fabric in the same manner, starting with a layer of 1.5-ounce mat, then a layer of twill, and finally a second layer of the 1.5-ounce mat. Compress each layer and roll out air bubbles before adding the next coat of resin.

4. Trimming the finished form. When the final coat of resin has become rubbery (after about 45 to 60 minutes), trim away the excess laminate along the mold edge with a utility knife; don’t lift or bend the laminate. Let the laminate cure overnight or longer, to lowing the manufacturer’s instructions, in a warm, dry, dust-free place.

5. Releasing the mold. Insert thin wedges of wood or plastic between the laminate and the mold at 6-inch intervals. Tap the wedges gently with a mallet. If the laminate does not spring free, don’t try to pry it loose. Instead, invert the mold and run warm water into it to melt the wax parting compound.

Finish the rough edges of the laminated basin by filing them with a forming tool, then sanding with 600-grit emery paper. Working from the inside of the basin, drill a drain hole with a carbide-tipped hole saw fitted in a power drill. Put a wood block under the drain-hole area while drilling, to avoid splintering the laminate.

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The many variables involved in fiberglass lamination can cause some problems in working with the resin and some flaws in the finished product. In most cases you can trace the source of the problem fairly easily and correct the damage.

Errors in measuring and mixing the resin can produce a resin mixture that does not harden. This may be the result of adding too much or too little hardener, or of using resin or hardener that is too old and no longer effective. If the resin fails to set on only part of the work, probably it has been unevenly mixed, or the mold or parting compound was not al lowed to dry completely. If resin does not cure after 3 to 5 hours, you will have to scrape it off, thoroughly clean the surface, and mix new resin.

A workshop that is too cool can also delay curing: Keep the temperature at 65° or above. But a room temperature that is too high—above 85°—can cause the resin to cure too quickly or to harden in the mixing pot before it’s applied. Mixing too large a batch of resin can have the same result. Activated resin generates he-at as it cures, and large quantities pro duce enough heat to make a significant different in hardening time. Left open too long—around 45 minutes—a large quantity of activated resin may even be gin to smoke, becoming a fire hazard.

Some errors are apparent only after the laminate has cured; they manifest them selves as shown in the photographs at right. Most can be remedied by the application of a second gel coat over the surface. However, in the case of leaching, it’s necessary to cut away the flawed area and replace it with a fiberglass patch. The same is true of any area where the layers of fiberglass fabric separate—or delaminate—and are no longer solidly bound together with resin.

--Crazing. Fine hairline cracks may appear in the surface immediately or as much as several months later. They are caused by incorrect proportions in the resin mixture or by the use of a laminating resin that is not compatible with the gel coat being used.

--Fiber pattern. A raised honeycomb effect on the laminate surface is the result of failure to use a layer of fine surface-mat fiberglass between the gel coat and thick woven roving. If the surface is smooth but the fabric pattern is visible, the gel coat is too thin or, in a molded form—where the gel coat is applied first—the fabric was laid on before the gel coat was stiff enough.

--Pin-holing. A pocked surface results from the introduction of tiny air bubbles into the gel coat, usually during mixing. Bubbles are particularly difficult to see in a tinted gel coat. Also, if the gel coat is too viscous, the bubbles, though visible, are difficult to remove.

--Leaching. Fiberglass fabric is left exposed when resin is washed away, either because it was improperly cured or because the resin used is not weatherproof. The fragility of the exposed glass in a leached area substantially weakens the laminate; the laminate should be patched.