Drain the tub and clean the area of the crack with a sponge and a mild cleanser. Scrub gently to remove all soap scum and oil build-up. Rinse the cleaner thoroughly and allow the tub to dry overnight.
Locate the crack and gently sand the area using fine-grit sandpaper. Rub in a circular motion to remove any rough spots and uneven edges. This will help the fiberglass hold the epoxy.
Drill a small hole into the ends of the crack with a drill bit that is 1/4 inch or smaller diameter.
Spray inside the holes with a small amount of foam insulating spray. Spray just enough to lightly cover the inside of the crack. The foam will expand and cause the fiberglass to bend if you apply too much.
Allow the foam to dry overnight. Sand down any foam that has came through the crack.
Apply a two-part fiberglass repair product to the hole. Mix the filler and hardener together and press it into the crack on the tub. Allow the repair to dry overnight. Match the color of your tub by adding two to three drops of acrylic paint to the repair mixture.
Sand down the area patch, and spray on a thin layer of epoxy or clearcoat.