Fibreglass bathtubs can last for a long time. Once they get a crack or a hole in them, there may be structural damage if you don't see to the repair as soon as possible. You can purchase a fibreglass repair kit if your fibreglass bath repair is minor. Another option is to purchase the bathtub repair supplies separately. This may be a better choice if the fibreglass bathtub repair job involves structural repairs as well as cosmetic ones.
Access underneath or behind the bathtub to begin the fibreglass bath repair project. You may need to take out drywall or flooring to reach the spot.
Cut a piece of plywood slightly larger than the crack or hole. Attach the plywood to the back of the bathtub with epoxy glue. This allows you to give the bathtub more structural support so that the tub doesn't crack again. Allow the glue to dry.
Use a handheld rotary tool with fine-grit sandstone to create a bevelled area around the crack or hole in the bathtub. Work carefully so as to not create any more damage to the bathtub. Wipe all of the grit out of the bathtub when you complete this step.
Cover the entire area of the bathtub except the area you are repairing with a plastic dust sheet or similar protection. Use masking tape to seal off the protected area.
Cut a piece of fibreglass cloth large enough to cover the entire bevelled area you are repairing. Set it in place. Cut progressively smaller pieces of fibreglass cloth until the hole or break is full. Pull the pieces back out and set them aside.
Mix fibreglass hardener with fibreglass resin in a small container using a mixing stick. Coat the largest piece of fibreglass cloth with the mixture and lay it in the hole. Repeat with each piece of cloth until the hole is once again full. Allow the cloth to dry.
Sand off any rough edges on the repair with fine-grit sandpaper. Remove all traces of the grit before proceeding.
Paint the repaired area with your choice of a gel coat or paint. You can usually find colour pigments to add to the gel coat or paint to match your bathtub. Allow the paint or gel coat to dry. Remove the masking tape and dust sheet.
Use goggles and a dust mask when sanding fibreglass. Open windows or run the bathroom vent fan when using paints.