May 11, 2021
What should I know about fiberglass repair?
There are a few fundamental principles that, once understood, allow most composite repairs to be completed successfully. Successful repairs allow you to extend the life of a part or mold and save you the cost of replacing your composite part. Start by considering these three main principles of composite repair:
Repairs differ from the original part — The first principle you must understand is that structural repairs are made by a different process than the original piece. When a composite part is initially manufactured, its resin cures bonding both chemically and physically with the reinforcement fabric resulting in a single unit, regardless of the number or orientation of the plies of fabric. This is referred to as the primary structure or bond, and it is the strongest type of bond that can exist within a composite part.
Once a part is damaged, all repairs become secondary bonds attached to the original primary structure. This means all repairs are dependent upon physical bonding to the surface of the original primary structure (more on this later). For this reason, fiberglass repairs rely upon the adhesive quality of their resin for their strength—the strength of the physical bond to the primary structure. Because of this, the resin used for the repair should be just as strong as the resin used to fabricate the part. In fact, resins with strong adhesive properties are sometimes used for repairs.
The increased surface area will increase the strength of composite repairs — Since fiberglass repairs depend upon surface adhesion (physical bonding) of the repair to the primary structure, increasing the surface area of the bond line will increase the strength and durability of the bond—and by extension the part or repair.
Typically, the method employed to increase the surface area is taper or scarf sanding. This type of sanding means the area next to the damage is sanded away gradually, generally resulting in approximately ½—¾ of an inch of area per ply of composite laminate. Scarf sanding is usually done with a high—speed compressed—air power sander. Since most composite structures are fairly thin, this is a gentle process. The size of the taper, relative to the thickness of the laminate, is expressed as a ratio. Generally, the stronger or more critical the repair needs to be, the larger the ratio. Structural repairs usually require a gentler taper, with a ratio of 20:1 up to 100:1.
An alternative method employed to increase surface area is step sanding. This procedure defines the size of the inner repair, then removes surrounding materials at a width of ½" per ply of the part, working towards the part surface. This results in considerable growth of the repair surface and allows the fiber orientation to be evident in each step.
Both methods are acceptable for the majority of composite repairs, though most consider scarfing to be easier, and it’s generally considered better. Stepping results in abrupt edges and butt joints in each repaired ply. It is also hard to step sand without cutting through, potentially damaging the underlying plies.
You should match your repair to the original part when possible — While your composite repair is different than your original part, it is recommended that you duplicate the thickness, density, and ply orientation of the original laminate when making your repair. This will help to maintain the functionality of the part. More is not always better—in this case, if your repair is thicker than the original part, it will almost certainly be stiffer, regardless of the material in use. Introducing various strengths within a part can cause unintended stress points, eventually leading to material fatigue or failure. It is better to carefully replace every ply that has been removed in the damaged area with identical material, placed in the same orientation when possible. This ply—for—ply replacement approach guarantees the repaired structure can withstand the same loads as the original, and that it will disperse loads as intended.
With these three fundamental principles in place: Repairs Differ from the Original Part, Increased Surface Area Increases Strength, and Repairs Should Match your Original Part; you have the foundation for understanding the sequences for repairs. As stated earlier, we are going to provide detailed steps for both structural fiberglass repairs and general cosmetic composite repairs. Keep in mind that these are general steps, and certain applications may call for additional steps or slight variances in approach.