Fiberglass Repair Instructions
Structural Repair Sequence
1) Inspect the damage, identify the materials involved, and determine whether the part should be replaced or repaired.
Damage generally falls into four categories, but they can all be present if the impact was severe enough. Tears, holes or punctures, crushed cores, and delamination are the most common problems. To make reference quicker, use a marker to outline the boundary of the damage. Close inspection is necessary as the problem area often extends farther than can be easily detected visually. A coin tap test is an effective way to test the surrounding area. An audible difference is easy to detect between solid laminate and crushed material when tapped with the edge of a coin.
Identification of the materials involved, especially the resin, can be more difficult. Begin by determining the type of fiber reinforcement used. This will often give clues as to the type of resin employed. Marine applications where chopped mat is most prevalent probably used general purpose polyester resin.
A new composite material is gaining widespread use in the automotive, industrial, and personal watercraft markets that presents unique repair problems. It is called SMC, or Sheet Molded Compound. Recognizing SMC from other types of FRP composites is critical so the proper repair can be performed. Parts made with SMC are produced in compression molds, so they are smooth on both the inside and outside. That is the first clue to look for when identifying them. Next, SMC parts do not have an outer gel coat, but they are usually painted or color molded. When the paint is sanded off, the underlying surface has a marble appearance. Finally, when damaged SMC is sanded, short coarse fibers are exposed and a dryer powdery dust is produced compared to conventional materials. These hints will make SMC identification quite straightforward.
SMC is a polyester-based material, but it cannot be repaired with polyester resin. This is due to the mold release agent that is present throughout the entire SMC part. Unlike conventionally molded parts where release agents are applied to the mold surface, SMC is compounded with them in the resin mix for quicker processing. This means that as the damage is sanded to prepare a good bonding surface, fresh mold release agent is exposed. Polyester resin products are not strong enough to adhere to this surface. SMC SHOULD ONLY BE REPAIRED USING EPOXY-BASED RESINS, FILLERS AND ADHESIVES. When painting, use only catalyzed type paint systems.
Once the extent of the damage and the type of material used is known, determine if the part should be repaired or replaced. If the manufacturer's specifications are available, check whether the damaged area is too large to be repaired. If no information can be reviewed, make a quick estimate of the materials and labor time needed for the repair and compare the figure to the price of a new part. A savings of at least 50 percent is the typical cut-off point to warrant proceeding with the repair.
2) Remove the damaged material and prepare the area for bonding.
Try to identify the sequence and the orientation of the material as it is removed layer by layer.
When a part is broken or crushed it is difficult to realign the pieces with each other because the frayed fibers tend to "hang up" on one another. Use a saw blade to cut the length of cracks or tears. This relieves the stress on solid laminates, which often return to their original shape with little force.
Cored parts tend to pancake and mushroom, further complicating realignment. A router is excellent for removing damaged core material without disturbing intact face skins. Try to remove as little material as possible so the repair does not grow too large. However, solid laminate must be exposed for a good repair. Continue coin tapping and grinding until all the damaged material is removed. Finally, determine the density of the core so it can be replaced with the same material.
Next, support the part so nothing gets distorted during the repair process. This can be as simple as strips of 2 inch wide masking tape, or as elaborate as a custom-made clamping fixture. Generally, high-performance parts require very precise support systems.
With the part supported, proceed to preparing the bonding surface by grinding a taper or steps around the damage. This is the critical step for functional repairs, but it is also the most overlooked and abused. If a taper is to be used, measure the depth of the valley and calculate how far the sanding must extend to achieve the desired ratio. Mark the outer edge of the taper using a marker, and begin sanding inward toward the valley. Be sure to remove material slowly so the taper progresses evenly. Write down the orientation and type of fabric used as each layer becomes exposed so it can be replaced in the same way.
When step sanding, the initial calculation becomes more precise. Assuming the damage is circular, 2 inches in diameter and there are 5 plies in the laminate, mark concentric circles expanding 1/2 inch per ply from the edge of the innermost circle. The final diameter of the prepared area will be 7 inches. Begin sanding in the center until the deepest layer is exposed. Step out 1/2 inch and sand down to the second deepest layer and so on, until all five steps are prepared. A right angle grinder offers the best feel and control for this delicate procedure.
For personal safety and cleanliness, tape the hose of a shop vacuum to the work surface so dust can be removed while grinding is taking place. When the sanding is complete the whole surface needs to be thoroughly cleaned. Vacuum any remaining dust and then wipe the surface with a solvent rag. Acetone is usually sufficient for removing oils, greases, dust, waxes, or other contaminants which would interfere with repair adhesion.
3) Laminate the repair patch.
Begin this procedure by precutting the core replacement and the reinforcement plies that will fill the repair taper. Cut the core first to fill the deepest hole. Some cores like Nomex honeycomb have their own orientation which needs to be aligned in the part. Be sure to check this. Consult the list that was made during the preparation process so that each reinforcement ply is cut to the proper orientation. Cut each ply so it fits precisely into the step that was prepared for it. Modern repairs are made ply-by-ply, so the smallest piece is intended to go into the bottom of the valley. Stack the reinforcements near where they will be used, with the first layer to be placed in the bonding area on top of the pile.
Mix the appropriate resin or adhesive system for the repair patch. Pre-weight the reinforcement schedule and mix only the same weight of resin. This will keep the resin content within a reasonable 50:50 ratio.
Pre-wet the entire bonding area with resin, then begin saturating each ply of reinforcement before it is placed into its step. Work on a sheet of plastic so the fabric can be easily lifted from the table once it is ready. A flexible rubber squeegee is the best tool for spreading the resin evenly through the fabric and removing excess which may be present. Place the reinforcement into its spot on the repair, ensuring the proper orientation. Stop every few layers to compact the patch as much as possible. A squeegee or grooved roller works well for this. Continue stacking the repair plies until all the fabric that was removed has been replaced. A final cover layer is then added over the entire area.
Plan on compacting the final repair patch as densely as possible while the resin is curing. Vacuum bagging is the most uniform method, but squeegees, rollers, or other clamping pressure will work adequately. Follow the manufacturer's recommendations for curing the resin or adhesive. If a two-sided repair has been planned, preparation of the opposite side can begin once the first patch has cured.
4) Inspect the repair before putting the part back into service.
Use the coin tap method to inspect the cured repair. The entire structure should resonate the same solid sound. Non-destructive load testing can also be used for greater confidence. This consists of stressing the part up to its expected service limit but not beyond. If the part fails prematurely, it should be discarded. For more critical structures, a testing laboratory or specialized equipment may be necessary before sign-off. If all is well, proceed to the cosmetic repair sequence.
"Build a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life!" - Anonymous