The correct way to effect a good auto fiberglass repair is to first check the damaged area. If you want your repair to look showroom quality you will be right to start with the correct preparation and use of materials. Good preparation is always the key to an excellent job.
Locate the damage to the fiberglass in your vehicle. Fiberglass car parts can have unseen damage and the best way to test them is to take a wrench or thick screwdriver and tap around the body work. Undamaged fiberglass will sound tight like a drum or dull metal. Damaged fiberglass will sound dense. It is similar to the difference between damaged pottery, where you get a ring sound if it is in good condition but a dull thud if it is not. Listen for the difference to find any damage that’s not obvious. If there are fiberglass cracks you will get the same result.
Measure the extent of the damaged area. Use a grinder to cut an angled piece away from the area, incorporating the piece that needs replacing. Make sure you create a tapered angle to that the piece you put in to replace it will slot in and not disappear into the hole in the body work. Remove the damaged piece and use that as a template to measure the exact size of the new piece. Always measure the widest parts of the piece. It is easier to trim excess away and make the piece smaller. You will need to use a solvent to de-wax the edges of the cutaway piece to offer better bonding. Place a waxed backing brace over the inside of the hole so that the area is easier to work with. Apply paste wax around the edges of the hole. Apply gelcoat to the backing brace.
Fiberglass replacement patches and mats need to be cut to fit the hole. Start with two layers of the mat and then two of the fabric. Place in one mat and one fiberglass and then alternate the layers, one each at a time. You should ideally use no more than four layers depending on the thickness of body work. At this point you can opt for epoxy or polyester resin to seal the patch. If you choose to opt for polyester you will also have to use laminating resin to increase bonding strength. For polyester you will also require a catalyst to help set the resin. This comes in the form of MEKP (methyl ethyl ketone peroxide), but be highly cautious when you use it. It has strongly corrosive properties.
Apply the use of a resin roller to smooth out the mended area. Smooth in a perfect line with the original body work. Once this process is complete you can remove the backing brace and fill any tiny spots or areas which require extra attention. You can then smooth over the entire area with sandpaper and follow normal primer and paint procedures to finish your project.