Hood Repair–Driver’s Fender Corner
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.
Fender corner damaged by cracks
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.
Fender getting epoxy and glass
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.
Trimmed after cure
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.
Liquid now, Hard later
Most of the traditional Thermoset Unsaturated Polyesters, Thermoset Unsaturated Vinylesters, and Epoxies arrive at the fabrication shop as liquids in buckets, drums, totes, or tankers. They require liquid hardeners to be mixed in at specific ratios in order to transform into a solid physical state. The esters use peroxide catalysts in the 1% to 3% range, and the epoxies use a specifically formulated part B designed in tandem with the part A manufacturer. The esters also can come prepromoted or unpromoted, which basically is another part of the chemistry package typically containing a cobalt-based mixture that promotes the curing process. Where this is added depends upon several variables, and can also affect the shelflife of the product.
Nontraditionally the Epoxies and esters can come into the fabrication shop as prepregs. These are resin and glass mixtures that are formable to shape but have all of the chemicals and glass mixed in, and are activated by heat (ovens). This of course requires that they be transported and stored in refrigerated containers. There are several tradoffs for prepregs including cost, delivery, process control, shelflife, etc.
Role of Gelcoat
Gelcoat is the decorative surface found on fiberglass parts such as boats, bathtubs, and restaurant seats. This outer layer is needed for aesthetics and protection of the underlying laminate structure. Chemically it is unsaturated polyester/vinylester resin that is unreinforced but heavily filled with a complex variety of additives. These additives are used to determine its color, UV stability, and chemical resistance.
The manufacture of fiberglass parts typically requires a gelcoat layer to aid in the release of the parts from the mold. The parts that will be painted also are manufactured with gelcoat but it is a “sandable” variety. These parts are often automotive-related, and may include pickuptruck toppers, fiberglass kitcar bodies, and other aftermarket add-ons such as running boards or hood scoops.
The unsaturated polyester/vinylester gelcoat is directly compatible with resins and laminate of the same chemistry. Gelcoat can be used with epoxy resins but requires a tiecoat for adhesion because of the chemistry difference.
Gelcoat is applied at 18 to 25 mils (thousandths) and will lose thickness as it cures. Typically when it is dry on the part, it can be down to 12 to 18 mils. This is a lot thicker than a painted surface. One of the disadvantages of gelcoat is that it can be more difficult to repair, especially with color matching. The thickness typically allows for sanding out scratches and blemishes, but going through can be painful and will require a respray. The outer layers of gelcoat will likely discolor over time due to UV degradation, and sanding and buffing into the underlying gelcoat may cause discoloration as the layers are different colors depending upon the depth.
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