Posted June 9, 2017 by Jefferson Bryant Know How

How to Repair Fiberglass Body Panels

Fiberglass is a fickle mistress. While amazing things can be created using a little bit of resin and fiberglass strands, it remains in a constant state of hardening. Sure, once the initial cure is completed, this process slows down significantly, over time the resin gets brittle. In the automotive environment, this means cracks, breaks and splits in the body of your Corvette. Add to that some of the more “adventurous” owners of many of these cars, you may have to repair fiberglass panels to get things back to normal. Take for instance the 1967 Corvette convertible we have here. In the 1970s, this was a cool show car with a deep purple metal flake paint job and an 8-71 blower hanging out of the hood.

The car had been sandblasted, which is a huge no-no for fiberglass, but moreover, the glass itself was in rough shape. Lots of cracks and breaks that needed repair.

Surveying The Damage

Throughout this car’s life, there have been some bumps and scrapes which have caused some damage to the body. One of the worst was a split under the rear deck on the rear firewall between the gas tank and the interior of the car. A previous owner had attempted a fiberglass repair, which we were able to remove with just the twist of a screwdriver. This needed to be fixed the right way. We also wanted to repair the original stinger hood that had been modded with a giant hole for the blower.

You can see a large vertical crack in the body. This needs to be repaired before moving on with the build of the car, or it will get worse.

The previous owner had attempted a repair on the underside, but this just popped off with a screwdriver, not good. Fiberglass is not an adhesive; it just doesn’t work that way.

Repairing A Fiberglass Crack

There are several factors that go into making a good fiberglass repair. The key is locking the new material to the old. While cleanliness is paramount, that is only half of the equation. Even with the surface clean and prepped, a mechanical bond is needed to secure the actual repair. This is best done by drilling a series of holes around the perimeter of the repair to lock it in.

After cleaning the area with an 80-grit sanding pad on a die-grinder, a series of holes were drilled into the body on either side of the crack. These are ½” holes. It is also a good idea to drill the ends of the crack to stop its progress. The holes will connect both sides of the repair, locking it all in place.

You don’t need any special tools to mix fiberglass resin, but you do need to know how to mix it. Polyester resin is a 2-part substrate that when properly mixed yields a work time of about 15 minutes. The hardener, MEKP (Methyl Ethyl Ketone Peroxide), is added in very small amounts compared to the resin base. Mixing small batches typically means counting drops from a bottle. There is a proper ratio based on room temperature of 76 degrees. The mixture is exothermic, which means it puts off heat as it cures. When the ambient temp rises or falls from that point, hardener is either reduced or added to ensure a proper cure. Too much hardener makes the resin brittle and it kicks too fast, not enough and it might not kick at all. Resin age is important as well. Fiberglass resin (the base) has a shelf life of about 6 months. It can be used beyond that, but more hardener is required.

Mixing resin requires paying careful attention to the hardener ratio. If you don’t have a syringe, you can count the drops. 12 ounces of resin takes around 25 drops, so you will be counting a lot and it is easy to lose count.

Only mix what you can use in about 15 minutes. When left in the mixing cup, the exothermic nature will speed up the curing process, rendering your mix into a solid block very quickly. On the car, when it is spread out, this can take an hour. A ratio of 2% hardener to resin is ideal, with 1% being a slow cure for hot days and 3% being fast for cold days. Don’t go outside of this ratio if you want a quality repair. Measuring by weight, 100g of resin would need 1ml of hardener. If you are mixing large amounts of resin, a syringe is advisable for measuring the hardener. For reference, 1 teaspoon equals 5ml.

The type of fiberglass mat you need for your repair makes a difference as well. There are several types of matting available, the most common being woven, chop mat, and loose fibers, all available in different weights. Chop mat, which has no pattern, is just a series of 1-2 inch long strands that have been pressed together into sheets. This is the strongest type of mat, but it does not lay down very smooth and it can come apart easily. This is good for large, flat areas where strength is needed. Woven mat is excellent for a top layer over chop mat for a uniform finish and heavy curves. Loose strands are best for building up a backing area, filling voids and for mixing your own reinforced body filler (think Duraglas® or “kitty hair”). For these repairs shown here, both woven and chop mat were used. The weights of the material have to do with the intended use. For automotive body repairs, the typical mid-weight mats available in any NAPA Auto Parts Store are perfect. If you are building your own body panels, then you will want to do more research.

First, the body was treated to a base coat of resin. This serves to hold the mat in place.

The brush was used to push the resin into the mat. The mat will become transparent when it is fully wet. This was layered three times.

The process was repeated on the underside of the body, completing the repair.

Repairing A Fiberglass Hole

Sometimes you need to fix a mod that was done by a previous owner. This stinger hood was modified so a blower could stick out, but that is not the direction of this build. Rather than buy a new hood, we opted to fix it. Note the holes have been drilled around the perimeter.

The top side of the repair was taped off, and a piece of sheet metal was used to form the crest of the hood. We bent the metal in a brake to get a crisp line down the center. The underside of the metal was masked off as well. The panel was taped to the hood.

We started with a layer of woven fiberglass to be the top layer.

With the woven layer on, we continues the process with chop mat, using a fiberglass docking roller to push out any trapped air bubbles. This adds a lot of strength to the repairs. We used about seven layers of mat for this repair.

The end results of the repair matched the crest of the original hood, and the repair is fully locked in place and ready for finish work.

Finishing

Finishing the repairs are just like any other fiberglass body work project. Sanding the repair smooth and then using a body filler and high-build primer to finish it. One extra step should be taken, which is to use a sprayable polyester primer. This seals the fiberglass much like a gel coat for a factory-perfect finish.

The last step after the bodywork is the polyester primer. This stuff goes on thick and is then sanded off smooth. 3-4 medium wet coats get the job done. This is over the filler work we did on the hood, but before the main body work to the rest of the car. The polyester primer has to be block sanded.

After paint, the hood is perfect, you would never know that there used to be a giant hole in the center.

The process shown here can be used for any type of fiberglass structure, including boats, RVs and campers, even shelters. The key to a good fiberglass repair is clean preparation, patience, and good quality components such as 3M resins and matting.

Check out all the paint & body products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to repair fiberglass, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Jefferson Bryant View All

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.