FRP Repair

Posted on 10/7/2013 2:10:00 PM By Sandy Niks

Today’s topic is a brief review of techniques used in repairing bonded fiber reinforced plastic (FRP).

This includes bonding inner to outer panels, as in closures (hoods, doors, decklids, etc.), bonding body panels to the frame, as well as repairing just the outer panel.

The first step is to assess the damage – is it just a cosmetic scratch that can be repaired with a body filler, or does the damaged area need to be removed before a repair can proceed. Note that cracks often extend further into the FRP than may be readily apparent. Remember to use all recommended safety precautions as the FRP materials are often considered irritants, at a minimum, especially when sanding, grinding, or handling parts with exposed fibers.

One method used in repairing FRP is to rebuild the panel using the fiber matrix, whether glass or carbon, along with an impregnating resin similar to what was used in the original panel. Multiple layers may be needed, along with pressure, heat, and vacuum to thoroughly “wet” the fibers, eliminate any potential air entrapment, and cure the resin.

Another method, used in some specific aerospace applications, is to use a “patch” with rivets.

Repair techniques using adhesives are probably the most common, especially for automotive FRP applications. The surfaces to be bonded need to be cleaned. An adhesion promoter or primer may also be needed.

FRP does not generally deform to form dents – it usually fractures and delaminates. However, if minor cracking or a “dent” is perceived, and the location is cosmetic, then a body filler (or pit filler) could also be used in this type of repair application, similar to the scratch repair noted above. This repair technique may also work for small punctures.

For more severe damage, the entire body panel or a subpanel could be removed and a replacement piece attached using aftermarket adhesives, along with any mechanical fasteners specified in the original design and manufacture. The original bond areas would most likely be used, after appropriate surface preparation. An example of a subpanel would be if a body panel was damaged below a window. The original body panel included the area entirely surrounding the window as well as the panel below the window as a single piece. The replacement would be cut to only replace the bottom portion, leaving the bonded area around the window intact. This technique may cause less damage to the vehicle than attempting to remove and replace the entire panel.

For some repair situations, the damaged area may be cut out, a supporting panel bonded to the interior surface, then a piece of the panel matching the contours of the exterior surface would be bonded into the hole and to the supporting panel.

The type of adhesive most commonly used in these repair procedures is a two-part epoxy. If the repair shop works on a lot of FRP repairs, meter and mix equipment similar to the production line could be justified, although the application of the adhesive would most likely be manual, not robotically applied. For smaller, more limited repair applications, adhesive applicators will be a single unit having two chambers already loaded with the respective adhesive resin and curing agent. When pressure is applied, the two parts of the adhesive are forced into a mixing chamber at the proper ratio before the bead is extruded onto the bond surface.

Although FRP is most commonly made using thermoset resins, thermoplastic resins may also be used for some applications. In addition to various adhesive repair choices, fusion techniques are also an option. The damaged area is heated and minor damage (small cracks, crazing) is “healed” by the resin flowing back together.

Additional information on FRP repair is provided in the lists below. Note that the lists are not intended to be complete nor an endorsement of any company.

Repair procedures:

3M Panel Bonding Repair  

Lord Adhesives FRP Repair Procedure

Marine repair techniques

More marine repair techniques


Adhesive suppliers:


Loctite™, Henkel  





Handbook of Adhesive Bonded Structural Repair, Raymond F. Wegman and Thomas R. Tullos, Noyes Publications, 1992.

Joining of Composite Materials, edited by K. T. Kedward, American Society for Testing and Materials, 1981.

Plastics Engineering Handbook of the Society of the Plastics Industry, Inc., 4th edition, edited by Joel Frados, Van Nostrand Reinhold Company, 1976, p 498.

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