Fiberglass can be repaired if there's a crack. Fiberglass is a pliable, soft material that hardens and strengthens as it dries. Almost any fiberglass crack or fracture can be repaired. The work might be tedious, as there a quite a few preparation steps in order to make a strong repair. The time you will spend repairing a crack or fracture depends on the size or location of the repair. As long as you take your time and prep the area correctly, your repair will be even stronger than that area of your tank was before the break.
Plug your air compressor in and allow it to build up pressure. Plug your air hose into the compressor and attach your dye grinder. When you have sufficient air pressure, grind directly on the crack until the actual crack is no longer there. You will have a "valley" where the crack was.
Soften the edges of the valley by grinding both sides of the edge into a gradual slope going down into the valley. You want the valley to be as smooth as possible for the best results. Take your sandpaper and scuff the area. Make sure the whole area is dull and there are no glossy spots.
Cut a piece of your fiberglass mat just big enough to fill the opening and place it in the valley. Gradually increase the size of each layer that you place in the valley until the mat is slightly thicker than the rest of your tank's surface. This will ensure that you get the area back to its original thickness. Take the mat off of the repair and set it aside for now.
Pour the resin in your bucket. The amount of resin depends on your repair size, so start out with what you think you will need, and make more if necessary. Pour the catalyst in the resin, making sure that your mixture is 19 parts resin and 1 part catalyst. Too much catalyst will cause the mixture to set up quickly, and you may not have enough time to finish your repair. If you don't add enough catalyst, the mixture may take too long to set up and may not harden enough at all.
Stir your mixture for at least 30 seconds. It must be mixed well to properly set up. Make sure the area you are repairing is free from dust and debris, and use your paintbrush to wet the repair area with your resin mixture.
Start with the smallest piece of mat that you cut, and stick it to your wet valley. Saturate the layer of mat with your mixture, using your brush. Lay the next largest piece of mat on the area, and repeat the process of saturating and adding layers until all of your mat pieces are applied.
Let the repair sit until it is hard and cool to the touch. Sand the repair until it's level and smooth. Check the repaired area for weak or thin spots. Make sure the repair is level with the rest of your tank, and not depressed. If you find weak spots or a depressed area, add more layers and repeat the process until you get the desired thickness.