How to Lay Fiberglass Using Epoxy

Begin by determining whether the damage is cosmetic or structural.

Shallow scratches, surface crazing, small depressions, and cracks confined to the coating may only require opening, filling, fairing, priming, and painting. Fairing compound is intended to restore a smooth shape; it does not replace the strength of damaged fiberglass laminate.

A crack extending through the laminate, a hole, an impact-damaged area, or separated fiberglass generally requires new fiberglass reinforcement. Remove enough coating and damaged material to determine the true extent of the problem. Continue until the remaining laminate is solid and firmly bonded.

If the boat has a cored structure, investigate the core as well. Wet, rotten, crushed, or separated core must be addressed before covering the area with new fiberglass. Also correct the movement, unsupported structure, water intrusion, or other condition that caused the original failure.

Epoxy can cause skin irritation and sensitization, while grinding fiberglass and partially cured epoxy creates irritating airborne dust.

Wear gloves, eye protection, and protective clothing whenever handling resin, hardener, wet fiberglass, or sanding dust. Provide good ventilation, especially when working inside a boat. Use an N95-rated or better particulate respirator when sanding or grinding, and follow the epoxy Safety Data Sheet when selecting respiratory protection for vapors or confined-space work. Never use solvents to remove epoxy from your skin.

Cover nearby surfaces before grinding or mixing epoxy. Fiberglass dust travels surprisingly far, and cured epoxy is much more difficult to remove than uncured material. NIOSH recommends increasing ventilation, avoiding skin contact, using compatible gloves and eye protection, and following the product label and Safety Data Sheet.

Remove loose paint, gelcoat, filler, delaminated fiberglass, and any weak or contaminated material. Round the outline of the repair rather than leaving sharp corners, which can concentrate stress.

For a structural skin repair, grind a gradual taper into the surrounding sound laminate. A commonly recommended starting point is a bevel approximately 12 times as wide as the laminate is thick. For example, a 1/8-inch laminate would require a bevel approximately 1½ inches wide. The repair schedule may call for a larger area or reinforcement on both sides. WEST SYSTEM explains how a 12-to-1 bevel provides enough area for multiple fiberglass layers to transfer loads across a repair.

Use coarse abrasive tools to establish the shape, but avoid overheating the laminate. Finish the bonding surface by hand-sanding with fresh 80-grit paper.

For an open hole, install a temporary backing surface to support the wet fiberglass. A thin piece of plastic sheet, laminate, or another material to which epoxy will not readily bond can be fitted behind the hole and supported until the repair cures. Cover other backing materials with polyethylene film or appropriate release tape.

A good epoxy bond requires a surface that is clean, dry, solid, and thoroughly abraded.

If the area may contain oil, grease, wax, silicone, or other contamination, remove it before sanding so the sander does not spread the contamination into the laminate. Allow the surface to dry completely.

Sand the entire bonding area to a uniformly dull finish. TotalBoat specifies approximately 80- to 100-grit preparation for fiberglass, while WEST SYSTEM recommends 80-grit as a good mechanical bonding texture. Vacuum away all dust. Perform a final wipe only with the cleaning method and solvent approved by the epoxy manufacturer, and allow the surface to dry completely before continuing. TotalBoat Traditional 5:1 instructions and WEST SYSTEM surface-preparation guidance provide detailed preparation requirements.

Do not use solvent to remove amine blush from cured epoxy. Wash blush away with warm water, mild soap, and an abrasive pad before sanding.

Do not wait until the epoxy is mixed to begin cutting fiberglass.

Make a paper or clear-plastic template of the repair. Cut enough progressively sized fiberglass pieces to rebuild approximately the original laminate thickness. Dry-fit every piece and mark its order.

For a beveled repair, one common schedule places the largest piece against the entire prepared bonding area, followed by progressively smaller pieces until the center of the bevel reaches the thickness of the surrounding laminate. Follow the epoxy manufacturer’s or repair designer’s prescribed schedule when one is provided.

If separate pieces must join, allow the specified overlap. WEST SYSTEM suggests approximately two inches for general cloth overlaps. Round the corners of patches so they are less likely to lift.

Heavy fabrics such as 1708 build thickness and strength quickly but consume much more epoxy and can be harder to conform to tight curves. Lighter cloth may require more layers but can be easier to wet out and shape.

Arrange the fiberglass pieces in order and place all brushes, spreaders, rollers, gloves, mixing supplies, and peel ply within reach. Mask the perimeter and protect everything below the work area.

Check the expected temperature and humidity throughout the working and curing period. Choose fast, medium, or slow hardener based on the product instructions, job size, and conditions. Warmer temperatures shorten working time considerably.

For a first-time product or unfamiliar conditions, mix a small test batch and allow it to cure before beginning the actual repair.

Use only the resin and hardener intended to work together. Follow their exact ratio; different epoxy systems use different proportions.

For example, TotalBoat Traditional Epoxy uses five parts resin to one part hardener by volume. Its calibrated pumps dispense the proper amount with one complete stroke of each pump. Other systems may use a 2:1, 3:1, or different ratio.

Dispense into a clean plastic, metal, or wax-free paper container. Do not use glass or foam containers. Mix thoroughly while scraping the sides, bottom, and corners of the cup. TotalBoat specifies at least two minutes for its Traditional 5:1 system. Do not alter the ratio to change the cure speed, and do not thin epoxy unless the manufacturer specifically permits it.

Mix small batches. Epoxy produces heat while curing, and a large mass left in a mixing cup can become dangerously hot and cure very rapidly. Spreading the mixture into a shallow roller tray or onto the repair helps dissipate heat and preserves working time.

If a filler or thickener is needed, first mix the resin and hardener completely and then add the filler.

Apply a thin coat of mixed, unthickened epoxy to the prepared substrate. This wets the bonding surface and helps prevent a dry interface beneath the fiberglass.

If the surface is rough or contains small voids, a separate small batch may be thickened slightly with silica and spread very thinly to fill those irregularities. Do not use a heavily thickened mixture to saturate fiberglass—the cloth should normally be wet out with unthickened laminating epoxy.

On a vertical or overhead surface, it may help to let the initial epoxy coat become slightly tacky before placing the fiberglass. This helps hold the fabric without making it impossible to reposition.

Position the first piece of fiberglass without stretching or distorting its fibers. Apply epoxy with a brush, roller, or spreader, working from the center toward the edges.

Use only enough resin to saturate the fabric. Properly wetted fiberglass usually changes from white or opaque to a more uniform translucent appearance. White fibers indicate dry fabric. Shiny puddles or fiberglass floating above the surface indicate excess resin.

Use a plastic spreader or laminating roller to press the cloth against the substrate and remove trapped air. Work gently and deliberately. Excessive brushing or rolling can introduce more bubbles and disturb the fibers.

The fiberglass provides most of the laminate’s strength. Excess resin adds weight, heat, and brittleness without adding useful reinforcement. WEST SYSTEM’s hand-layup instructions describe the appearance of properly saturated cloth and the removal of excess resin.

Apply each additional fiberglass piece while the previous layer remains wet or tacky. Wet out each layer completely before adding the next.

Check continually for:

• White or dry fiberglass
• Air bubbles
• Bridging across corners
• Wrinkles or folded fibers
• Lifted edges
• Resin-rich puddles

Use a grooved laminating roller or plastic spreader to consolidate the layers and remove air and excess resin. Avoid pressing so hard that the laminate becomes starved of resin.

If the epoxy reaches a full cure before another layer can be applied, follow the manufacturer’s secondary-bonding instructions. This normally involves washing away any amine blush, drying the surface, sanding it to a dull finish, and removing the sanding residue.

Peel ply is optional but can make the next stage easier. Lay it over the final wet laminate and smooth it into the epoxy without adding an excessive amount of resin.

Once the laminate has cured, peel the fabric away. Properly applied peel ply leaves a textured surface, removes surface contamination with the fabric, and reduces the amount of sanding required. Inspect the exposed surface and follow the epoxy manufacturer’s instructions before fairing or recoating.

Protect the repair from rain, condensation, dust, movement, and temperatures outside the epoxy manufacturer’s limits.

Do not assume the laminate is ready to sand simply because it feels hard. Epoxy may become firm well before it reaches its specified sanding time or full cure. Premature sanding can clog the paper, distort the surface, and expose the user to dust from partially cured epoxy.

Use the cure schedule for the actual resin, hardener, temperature, and repair thickness.

If peel ply was not used—or if the product instructions otherwise require it—wash the cured epoxy with warm water and mild soap to remove possible amine blush. Rinse and dry it before sanding.

Sand the repair enough to remove sharp edges, high spots, and irregularities without cutting deeply into the structural fibers. Vacuum away the dust and inspect closely for voids, lifted edges, dry fiberglass, or areas that did not bond properly.

A significant void, soft area, or poorly bonded layer should be opened and repaired rather than hidden beneath fairing compound.

Once the structural laminate is sound, apply epoxy fairing compound to restore the desired contour.

Spread the fairing compound slightly above the finished surface and allow it to cure according to its instructions. Sand with a fairing board, hand block, or orbital sander as appropriate. Use a straightedge or flexible batten to identify high and low areas. More than one fairing application may be required.

Remember that fairing compound shapes the surface; it does not provide a substitute for missing fiberglass reinforcement.

TotalBoat TotalFair, for example, is mixed 1:1 until it reaches a uniform green color. Its working and sanding times vary substantially with temperature. The manufacturer recommends washing even this nominally non-blushing compound before sanding or overcoating. TotalBoat TotalFair instructions should be followed when that product is used.

Remove all sanding dust and apply a primer compatible with both the cured epoxy and the final coating. Epoxy should not be left permanently exposed to sunlight.

Apply topside paint, nonskid deck coating, antifouling paint, or another finish suitable for the repair location. If gelcoat is planned, verify the complete coating system carefully. Gelcoat generally should not be applied directly over cured epoxy without an approved epoxy tie-coat or barrier primer.

After the coating has cured, inspect the repair and test it under realistic conditions. For a deck or scupper repair, this may include a controlled hose test followed by an inspection from below. Continue monitoring the area after the boat has experienced normal loading, temperature changes, and vibration.