One of my recent projects was the repair of a large SMC fiberglass truck hood. It required several areas of repair, one of them including an outside wheel opening on the driver”s side of the hood. This area had a couple of large cracks from some sort of impact trauma.
I began the work by preparing the area by grinding away the outside layer of existing material. This helps us get a good mechanical bond and a clean surface. It also allows for a feather edge transition between the repair area and the existing composite.
The ground-out area is within the black outline created by a primer layer, and the underlying SMC is white in color. This area was ground with 40 grit sandpaper and followed by a healthy dose of compressed air to remove the dust from the process. I then wet-coated the areas with epoxy resin before turning to the glass and wetting that out. The soaked glass was applied to the back side of the fender to add strength back to the cracked laminate. A thickened epoxy paste was added to the front to fill the cracks that were ground out, as well as filling the holes drilled in the end of them.
Driver’s fender from the backside
The glass applied to the area was applied down with a strong bristle brush to get maximum surface contact and remove air bubbles between the layers. The strength of the glass and epoxy will restore this area of the hood back to its original specification as long as a good bond is made to the composite laminate.
As the resin and glass cure, there is a time window when the resin is in the solid phase, but not rock hard. This is when it is appropriate to take a utility knife and trim the edge of the loose glass. If it is tried too soon, it will move the glass on the laminate repair, and if it is too late, the knife will not cut through. If this time frame is passed, it still can be
cut with a saw blade or grinder, but is much messier. This is also a good time to wipe up any resin that is draining from the repair.
The repair area can now be sanded and body finished before paint and primer! This resin will need to be covered to prevent UV damage in the future.