I just spent 50 hrs. doing a glass repair to the rear of a 72 Boler
. a car caved the rear into the seating area. the space between the seats. Internal damage was to the short back wall between the seats. When the impact occurred the upper curves above the rear window on both sides was bend in and cracked (fractured ) the surface a foot wide and about two feet in height. I cut out the rear panel at the bumper height. ( bumper cut off). I ordered glass mat and resin. I ground the outside surfaces on an oval with the center almost cut thru. I bought an electric grinder from harbor freight it worked great very little sanding. To fill the areas in I started with a piece of mat one third the size of the oval ground out area. I didn't wait for the resin to set I cut another oval piece of mat half the distance of the first piece to the ground out area edge and placed it over the piece first laid up, and a third one over this one. I pulled the mat edge of the last piece to form a 1.5 inch feathered edge one the finished piece, that extender about an inch plus beyond the ground out area It brought the surface almost up to flush with the Gel coat surrounding the ground out area. The glass was so thin in the center of the fractured curves that it almost went thru. I fit the inside rear floor piece in place, I put an 8 inch backer on the inside of the rear shell at the hole, I screwed the back panel to a wooden form, that had glass mat, on top of wax paper, it was 1/4 inch piece of plywood, ( that was removed each time the resin set and a new layer of wax paper and mat attached so that it would bend when screwed onto the cut out shell piece and to the shell that remained in place. I screwed the cut out piece, on and off 3 times giving me 3 layers of mat 4 inches in on the trailer shell and 4 inches to be a backer for the cutout piece, that then had resin applied to it and it was screwed back onto the 1/4 inch plywood, with 3 layers of mat attached to it. ( the screws were removed when the resin was set)...I agree with DWhite in all respects. I put 2 layers on the inside of the fractured curves . I worked in my garage when the temperature outside was in the teens. I could heat the garage to 60 degrees. The hardener Can be added at a much higher ratio if the temp is below 70 degrees. A very few drops at 100 degrees. The resin is as hard as a rock and when it was painted the painter said the glass job was very impressive.