When you purchase your materials for the job most places will be non-commercial type stores. The fiberglass should be the fiberglass mat, not the fiberglass cloth. The cloth fiberglass has a uniform weave to it, much like a screen door. The fiberglass mat is not uniform in its appearance. The thickness of the mat is usually 11/2 oz.
The resin needs to be the right thickness--not too thin or not too thick. Too thin and it will run out of your repair. Too thick and you will have a hard time getting saturation. The most used grinding tools will be an electric drill, a 3" rubber pad, and 24 or 36 grit grinding discs. Other tools include 1" brushes, mixing cups, acetone, and above all, a dust mask. When you tear the fiberglass mat, thousands of glass fibers float in the air.
A basic repair could be a break in the fender. This type of break will ordinarily start from the wheel opening up over the fender to the hood gap. To fix the break, if it has gone clear through, first tack the fender into place with screws and tabs, or some other makeshift way to hold it into place. Go under the fender and grind the glass clean (very clean) about 4" on each side of the break. Take a 6" wide strip of glass and glass it into place over the break from under the fender. Repeat this process three more times using a 5" piece, a 4" piece and finally a 3" piece. Wait at least 24 hrs. before going on further.
Next, grind the top side of the fender directly on the break. Grind until you break through into the glass you put underneath the fender yesterday. Allow about a 1" wide strip of the underneath glass to show. When finished, the width of the gash now on top of the fender should be 2" to 3" wide. You have ground the original fiberglass of the fender this wide in order to get to the underneath glass. Now lay one strip of fiberglass in this area. It is best to tear the fiberglass instead of cutting it when doing any work top side. Resin-soak this strip of fiberglass. You should always have total saturation of the fiberglass with resin. The fiberglass will turn clear when you have achieved this. Roll out the air bubbles with the roller you purchased, or will purchase. Wait at least 24 hrs. The more sun on the project during the 24 hour wait, the better.
Now grind all the glaze from this repair leaving some of the glass strip in place. Be sure all the glaze is removed. Now glass another strip on the repair. Wait 24 hrs. and repeat this again. Usually this takes 3 to 4 times until you come above the surface. Grind the repair until the contour is equal with the fender. If you do all this glasswork the same day you risk the repair shrinking, and it will be seen in the final paint job. The longer curing time in the sun the better.
The type of repair previously explained I refer to as a radical repair, one that has broken clear through. This needs a lot of cure time. Once the car is all repaired and sanded totally clean, no finer paper than 80 grit, then you can gel coat the fender or car to secure the repair or repairs. (Refer to the subject "CORVETTE FIBERGLASS BODY REPAIRS" for points on the gel coating.) You may, if desired, use bondo under the fender to help blend in the underneath repair. I have made a rule to always glass both sides of a repair, even when you can't get to it. I go under the fender, or any place else, and cut a window out to allow me to do the underside of all repairs. I then replace the cover on this window using fiberglass and resin. The places that you don't glass on both sides will come through and show in the final paint job. I had this happen 2 years after one of my cars was completely finished.