Use a rotary tool to smoothen the surface of the damaged or worn out part of the tub. Grind away the bulging part of the tub around the damaged area and make a smooth bevelled surface. Remember to don a protective mask as the fiberglass particles are very small in size and can cause irritation in eyes. Use sand paper to smoothen the surface where epoxy is to be applied.
Buy two sheets of 80 grit sand paper and rub them hard against the fiberglass to prepare the surface. After sanding use a clean cloth to remove dust particles from the fiberglass. Rinse the surface with water and once again apply cloth to remove any remaining dust particles from the surface of the tub.
Purchase fiberglass repair tool kit from a nearby hardware store and cut few layers of fiberglass repair cloth to fix the damaged area. Layers must be cut in a way such that each upper layer of the cloth is slightly bigger than the previous one. The total number of layers depend on the damage done to the tub but generally 3 or 4 layers are sufficient for a normal sized crack.
Take out the epoxy fiberglass resin and mix it with the hardener. The mixing ratios can be found on the back of the hardener package. Mix the paste to satisfaction, then use the paint brush to apply the resin on the largest fiberglass sheet. Put the fiberglass sheet on top of the cracked surface and push it with your hands.
Wear protective gloves to protect your hands from the toxic resin. Apply the paste to the second layer of fiberglass and put it on top of the previous one. Keep repeating the process till you cannot see the crack anymore with your naked eye.