May 13, 2021
Since a structural composite repair will typically be followed by a cosmetic repair, we will start with the instructions for the more severe damage. In this section, we will answer the frequently asked questions of "How do I repair fiberglass?" and "What do I need to repair fiberglass". This will be broken down into 4 steps:
1. Inspect and measure, identify and determine
2. Removing the damaged material and prepare the surface
3. Laminate the repair patch
4. Inspect the repair
There are four main categories for composite damage, your repair may involve one or all of these categories depending upon the severity of the impact or failure. The categories are as follows:
Tear — This damage results when the tensile strength of the composite part has been exceeded and the laminate has failed. This typically results in a fracture that extends completely through the substrate.
Hole or Puncture — This damage is typically a result of an impact or cutting. Holes and punctures are sometimes limited to surfacing layers or skins.
Crushed Core — This damage applies only to composite parts containing sandwich core materials. It is typically the result of an impact that forces the composite skin of the laminate to deflect—but not fail—causing the sandwich core material to collapse.
Delamination — This damage results in layers of the material separating from each other. It is typically caused by impact or stress between or across the layers.
Once you understand what type of damage you have, we recommend that you use a contrasting marker to outline the boundary of the damage. This will help you to understand the scope and magnitude of the damage, as well as offer you the chance to fully inspect your part. Take note, however, that you should inspect the damage carefully as the problem area often extends farther than can be easily detected visually. One inspection tip we can offer is the coin tap test. By tapping a coin around the surrounding area, you can quickly and easily generate an audible difference between a solid laminate, a crushed material, and a potentially delaminated area.
Once the extent of the damage and the type of material is known, you should determine whether the part in question should be repaired or whether you’re better off replacing it. If you’ve discovered the manufacturer’s specifications while you were identifying the necessary materials, check whether the damaged area is too large to be repaired. If no information can be reviewed, we recommend that you make a quick estimate of the materials and labor time needed for the repair. Compare this figure to the price of a new part. Typically, if you can achieve savings of at least 50 percent, you’ve met the cut—off to warrant proceeding with the repair.
Assuming you’ve already inspected the damaged area and marked it with a contrasting marker, you’re ready to begin removing the damaged material. When a part is broken or crushed, it can be difficult to realign the pieces because frayed fibers tend to "hang up" on one another. In order to maximize the strength of your repair, you will want to identify the sequence and orientation of the material as it is removed layer by layer. To help with this, use a saw blade to cut along the length of cracks or tears. This will relieve the stress on solid laminates, often allowing them to return to their original shape with little or no force. Parts constructed with a sandwich core material tend to pancake and mushroom, further complicating realignment. A router is excellent for removing damaged core material without disturbing intact face skins.
When removing damaged material, try to remove as little material as possible, so the scope of the repair does not grow larger than necessary. Be sure, however, to remove enough material to leave yourself with a solid laminate, which is necessary for a good repair. As you are removing damaged material, periodically stop and conduct another coin tap test. This will allow you to ensure you have removed all damaged material, and you should continue grinding or cutting away until all damaged material is removed.
After you’ve removed the damaged material, support the part so nothing is distorted during the repair process. Sometimes this is as simple as affixing a few strips of 2—inch wide masking tape, other times it is as elaborate as a custom-made clamping structure. Generally speaking, high—performance parts have tighter tolerances and will require a more precise support system. Once you’ve got the part supported, proceed with preparing the bonding surface. As mentioned above, this typically involves grinding or sanding a taper or steps in the material around the damage. This is the critical step for functional repairs, but it is also often overlooked or abused. Take your time and ensure your repair is performed correctly.
If a taper is to be used, measure the depth of the valley and calculate how far sanding must extend to achieve the desired ratio. Use your contrasting marker to mark the outer edge of your taper and begin sanding inward toward the valley. Be sure to remove the material slowly so that the taper progresses evenly. As you expose each layer, write down the fabric type and orientation so that you can replace it in the same way.
When step sanding, the initial calculation is even more precise. For example, assuming the damage is circular, two inches in diameter and there are five plies in the laminate, mark concentric circles expanding one—half inch per ply from the edge of the innermost circle. The final diameter of the prepared area will be seven inches. Begin sanding in the center until the deepest layer is exposed. Step out one—half 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.
With either method, for personal safety and cleanliness, we recommend that you tape the hose of a shop vacuum to the work surface so that dust can be removed while grinding is taking place. Additionally, be sure that you wear a respirator and if possible protective clothing as this is a very messy process and you want to be as protected as possible. When sanding is complete, the whole surface must be cleaned thoroughly. Vacuum any remaining dust and then wipe the surface thoroughly with a solvent rag. Acetone is usually sufficient for removing oil, grease, dust, wax, or other surface contaminants which could potentially interfere with your repair’s adhesion.
Inspect the repair
Before you put your repaired part back into service, you need to inspect your repair. Use the coin tap method to inspect the repair after it has fully cured. The entire structure should resonate with the same solid sound. For more peace—of—mind or more critical parts, non—destructive load testing can also be implemented at this stage. This consists of stressing the part up to its expected service limit, but not beyond. If the part fails prematurely, even if the failure occurs away from your repair, it should be discarded. For more critical structures, a testing laboratory or specialized equipment may be necessary before sign—off.
If your part passes its inspection, you’re ready to proceed with the cosmetic repair.
A part’s outer cosmetic finish is designed to hide and protect the structural reinforcement below, while also being aesthetically pleasing. It is this surface that most people will look at and judge the whole structure. Even slight damage like scratches and gouges will mar the finished appearance. Worse, these often create a direct path into the structural layers, causing even larger problems. Cosmetic blisters are a problem which, left untreated, can turn entire boat hulls into sponges.
The cosmetic composite repair sequence is similar to the structural sequence, but fairing and filling compounds and/or gel coats replace the reinforcing material. Surface preparation is still the most important aspect of a long—lasting repair. Finish sanding and polishing present an additional step, but time invested in that step can result in a truly professional composite repair. Many people fear tackling cosmetic work because they assume it requires expensive spray equipment for satisfactory results. While equipment is important, proper material selection is the key to positive results.
In this section, we will provide guidelines for cosmetic composite repairs, including fiberglass crack repair, fiberglass gel coat repair, and composite repair finishing. This will be broken down into 5 steps, which are similar to those required for structural repairs:
1. Inspect, Measure, and Identify.
2. Removing damaged material and preparing the surface
3. Mix and apply filler
4. Polish the repair area
Now let’s dive into each of these in more detail.
Inspection of cosmetic damage is just as important as it is with structural damage. There are four main categories for cosmetic composite damage, and you may find one or all of these types of damage depending on what your part has been through. The categories are as follows:
•Gouge A gouge is a long, deep depression in the surface, severe enough to require filler and a surface coat for repair. Gouges are typically caused by rough handling, glancing impacts, or improper padding.
•Blisters Blisters are bulges appearing on the surface of a part. It is typically caused by liquid or gas that has gotten into the substrate, forcing the skin away from the glass.
•Scratches Scratches are similar to a gouge, but not deep enough to require filler. Scratches are typically able to be repaired without the need for filler.
•Crazing Craze cracks are hairline fractures that do not go through the thickness of the part, sometimes they do not even go through the thickness of the surface coat. Crazing is typically caused by either over-stressing of the part, thermal issues, or stress between or across the layers.
Once you understand what type of damage you have, we recommend that you use a contrasting marker to outline the boundary of the damage. Note that gouges often leave undercut areas of unsupported gel coat which at first will appear to be fine. However, if you press on them with a blunt tool, they will easily crack away. It is critical that all damaged areas are fully identified so they can be prepared correctly in the next procedure. Once you have your areas identified and marked, determine the approximate surface area that requires repair.
After you’ve successfully identified the size and scale of the cosmetic damage, you should be able to approximate how much repair material is required. Gouges and Blisters will require both filler and gel coat, while scratches and crazing can typically be repaired with gel coat alone. If your part was originally made with polyester resin, polyester filling compounds and gel coat are suitable for repair. If you are unsure, we also offer epoxy fillers.
For both gouges and blisters, you will need to be sure that unsupported surface material is removed. As mentioned previously, this can be accomplished by pressing on them with a blunt tool. Once unsupported material has been removed, wipe the surface with acetone and a rag to remove any wax, oil, or grease which might contaminate the repair.
When the surface has dried, tape off the area surrounding the damage. This will keep sanding scratches to non—damaged areas to a minimum. Proceed by chipping out all loose material with a utility knife and then use 40—grit sandpaper to bevel the edges to a taper. Even thin cracks will have to be "opened up" before material can be added to fill them. Blisters will have to be completely exposed at this time. While you are expanding the damaged area, we ensure that the remaining laminate is solid and dry. If the laminate is not solid and dry, you may have additional structural repairs to complete prior to finishing your cosmetic repair.
After you’ve removed the damaged material and opened up the cracks, use medium-grit sandpaper to sand slightly into the surrounding area. This will give you the ability to feather your repair into the undamaged surface. After you’ve finished sanding the surface, wipe down the surface with acetone one final time to remove dust and anything else that may disrupt adhesion.
Shallow scratches can be directly filled with a color-matched gel-coat rather than a filling compound. Be sure that you match your gel coat to a sanded and polished portion of the original, rather than the overall faded color. Your repair may stand out at first, but everything will fade to the same color in about a month. When using a gel coat to fill a scratch, use a small brush to dab in the gel coat. Be sure it is higher than the surrounding surface so that it can be sanded flush. Spray a light coat of PVA over the repair for a tack—free, sandable cure.
Deeper gouges will require structural repair putty to replace the missing material before the finish coat of gel coat is applied. Resin mixed with 1/32 inch milled glass fibers is an excellent structural putty. We also offer a variety of both polyester and epoxy—based fairing and filling compounds. Use a squeegee to spread your selected filler into the bottom of the gouge. Keep this filler slightly below the surrounding surface if the gel coat is the intended topcoat. If painting, the filler can be level with the surface since paint adds little thickness. When filling blisters, such as in a fiberglass boat hull repair, be sure to use vinyl ester resin with your milled glass fibers to make your putty. This will add additional corrosion protection to these weakened areas.
If your cosmetic repair is following a structural repair, it may be necessary to level the structural patch with the solid laminate. Both grinding and filling may be necessary to accomplish this. Grind all high spots until they are flush with the rest of the surface unless this will compromise the structural strength of the patch. Use filler to level any low spots. If grinding cannot take place for structural reasons, fillers can be mildly added to smooth irregularities and sanded to a smooth feather, but the patch will then always be visible. For marine repairs, use the 1/32 inch milled glass putty filler as described previously. Other structures can usually be filled with standard fillers or by mixing talc and a resin unless otherwise specified. Continue adding filler and sanding until the surface is perfectly flush.
Epoxy repairs are quite stable once they are cured, but polyester repairs should be heated slightly before final finishing. Polyester resin in the repair patch and in standard body filler often continues to shrink for some time when exposed to heat. This can be avoided if you use one of our non—shrink polyester fillers, but otherwise, simply place the project out in the sun for a few days or set up a heat lamp. If a lamp is used, however, don’t place the part too close or leave it for too long. You are trying to "force" the filler to shrink prior to final sanding and don’t want to distort your part. Once you’ve sanded to the perfect contour, your part is then stabilized against future heart problems.