Composites Repair

Composite parts are widely used due to their unique properties, including durability. Unfortunately, even the best designed and best made parts can be damaged by unforeseen circumstances either at the OEM or in service. The following steps cover best practices for repair of anything from gel coat gouges or scratches to damage penetrating through the entire laminate. Before starting, put on the appropriate personal protective equipment for the task, including protective clothing, safety glasses, gloves and a respirator. Step 1: Evaluate the work area. Make sure the work area is well lit, but not in direct sunlight. The temperature of the work area, part and all repair materials should be 75 to 80 F. Temperatures below 75 F extend cure times of repair materials and can result in under cure. Elevated temperatures speed up cure, reducing work time. For cool conditions, heat guns or heat lamps can be carefully used to warm parts and repair materials. The area is too hot if you can’t hold your hand on it. Patching materials are flammable, so be careful. Step 2: Determine the type and extent of the repair. Inspect the part to determine the extent of damage and type of repair needed, which will also dictate your materials and tools. For minor defects, determine if any repair is needed by standing a reasonable distance away from the part. If you can’t see the defect, a repair may not be needed. If a minor defect requires repair, a spot or dab patch with gel coat may be sufficient. Deeper defects may need to be filled with putty prior to covering with gel coat. For defects penetrating into or through the laminate, laminate reinforcements and resin must be replaced. Any remaining voids must be filled with putty. For gel coated parts, the area should also be spray patched. Step 3: Set up the equipment and obtain materials. Gather your equipment and tools. Inspect all equipment before use to ensure operator safety as well as part quality. If anything is missing or broken, repair or replace it before proceeding. Obtain all necessary materials. Make sure gel coats and resins have been thoroughly mixed prior to removing samples for repair. For repairs requiring gel coat, use the same batch originally used to spray the part whenever possible to achieve the best color match. When patching an older part, weathering may have occurred, so a tinting kit may be necessary. You can lightly sand and buff an area to see the true color. Step 4: Prepare the repair area. Remove the damaged material. For minor defects, hand sanding with a sanding block or a dual action sander may be sufficient. For laminate repairs, use a saw to cut away the damaged portion. Make sure to remove all damaged material so that the remaining material is structurally sound. Once the damaged material has been removed, prepare the repair area by grinding or sanding to facilitate adhesion of repair materials. Remove all dust, oils, waxes and other contaminates. Keep the repair area as small as possible to minimize finishing work. Mask off the repair area to prevent damage to surrounding areas. Step 5: Measure and mix repair materials. It’s critical to accurately measure all repair material quantities, including resins, gel coats, patch aids and peroxide initiators. These materials must be used within specified ratios to perform properly and allow completion of a successful and durable repair. The preferred measuring method is by weight. Low-cost scales can be purchased from local pharmacies, laboratory/hospital supply firms or scientific apparatus suppliers. The next best method is to measure by volume using graduated cylinders. Many repairs are small and require small amounts of patching materials. Be sure to prepare materials in quantities that can be measured accurately even if the majority of the material is discarded. It is cheaper to discard excess material than to redo a repair because of inaccurate measurement. For gel coat repairs, patch aids are recommended. Patch aids are resin- or monomer-based solutions that improve the working properties of gel coat when patching. They lower viscosity for easier spraying and reduced “orange peel” – or uneven surface texture. This reduces the time required to finish the repair. Patch aids also accelerate cure so that patches can be finished sooner and reduce surface tackiness for better sanding. Thinning gel coat with solvents such as acetone is not recommended. Solvents slow the cure of the gel coat and can result in off-color, dull and/or hazy patches. Step 6: Apply repair materials. For laminate repairs, use a roller or squeegee to pre-wet the glass. If working from the laminate backside, create the needed contour using cardboard or aluminum covered with cellophane prior to applying the glass to the repair area. If working from the front side, make a plug from cardboard with wires through the center, apply the glass to the plug and insert through the hole. Use the wire to secure the plug against the laminate until the resin cures. Remove air voids with a squeegee or roller. Allow the resin to cure thoroughly before proceeding. For gel coat spray repairs, use a precision touch-up spray gun to spray the catalyzed gel coat mixture onto the damaged area. Spray 8 to 12 mils wet thickness. If you are spraying an area where the gel coat was completely removed, apply gel coat until achieving good hide (minimum 12 mils) using more than one pass. For spot patching, use a knife or spatula to dab the catalyzed gel coat mixture into the damaged area. Slightly overfill the area, including the area around and above, to allow for shrinkage. Puncture and eliminate any air bubbles that may be trapped in the gel coat. Allow the patch to cure thoroughly. The patch is not cured sufficiently if a thumbnail leaves an impression in the gel coat. Step 7: Finish the repaired area. Initially sand with coarse grit sandpaper, followed by a succession of several finer grit sandpapers. The final sandpaper grit should make the patch flush to the part and remove scratches from the previous grit of sandpaper. Always use a clean shop rag to remove the sanding dust, oils, wax and other contaminates, especially when changing to a different sandpaper grit. Sand in one direction per grit size. For example, sand north/south with 400-grit and then east/west with 600-grit sandpaper. Remove any remaining masking. Buff using a clean 100 percent wool pad with medium grit rubbing compound to smooth the surface. Wash the area with soap and water to remove the rubbing compound residue. Use a clean wool pad and a finishing glaze compound for the final buffing to remove swirl marks. Thoroughly wipe the area to remove all traces of the finishing glaze and residue. Wash the area with soap and water before waxing. Wax with a UV-stabilized exterior-protective paste wax. By following these basic steps, you’ll be on the road to creating structurally sound, cosmetically appealing composites repairs.