To patch a crack in a fiberglass tub, clean and sand the damaged area, and apply adhesive, filler, epoxy, fiberglass cloth and sealant to the cracked area. Fiberglass repair kits typically contain the products needed to fix a cracked fiberglass tub.
Clean the tub with a household cleaner, rinse with plain water, and dry the damaged area. After cleaning, use fine-grit sandpaper, and sand the area around the crack gently. Remove dust with a clean cloth, and apply a fiberglass adhesive to the cracked area and the surrounding 1 inch. Allow the adhesive to dry, and fill the crack with fiberglass filler using a putty knife. Scrape excess filler off the tub immediately.
Allow the filler to cure for the recommended amount of time, and paint a layer of fiberglass epoxy over the cracked area and the surrounding 2 inches. Cut the fiberglass cloth so it covers the same area as the fiberglass epoxy, and place the fiberglass cloth on the epoxy before the sealant dries. Then, smooth the fiberglass cloth with the putty knife, and cover the cloth with a layer of epoxy. Allow the epoxy to dry, and sand the patched area to match the surface of the tub. Apply sealant to the patched area, and allow the sealant to dry before using the tub.