How to Fix a Hairline Crack in a Fiberglass Tub

Hairline, spider, and stress cracks develop on fiberglass tubs due to frequent use, insufficient support, use of harsh cleaners, dropping the tub, and hitting it with heavy objects. If not fixed on time, these small cracks develop into larger cracks which lead to leaks and eventually the bathtub breaks apart. Luckily, you can fix hairline cracks on fiberglass tubs quite easily. 

Clean and dry the cracked area then sand it with fine-grit sandpaper. Apply an adhesive on the crack followed by a fiberglass filler. Sand the area again then apply an epoxy mixture followed by a piece of fiberglass clothing. Apply more epoxy mixture then a sealer to fix a hairline crack in a fiberglass tub. 

How to Fix a Hairline Crack in a Fiberglass Tub

Both acrylic and fiberglass bathtubs are based on fiberglass and thus the procedure for fixing cracks on each is almost the same. You can thus use the steps below to fix acrylic or fiberglass tubs or even shower pans made of fiberglass.

How to fix a crack in a fiberglass tub 

To fix a crack in the bathtub, you need a household cleaner, fine-grit sandpaper or emery cloth, scissors, a cleaning sponge or rag, a dry lint-free piece of cloth, a small paintbrush, and a fiberglass tub crack repair kit. Make sure there is enough ventilation and you have rubber gloves on for this procedure.

Repair a crack in a fiberglass tub is as follows:

1. Clean and dry around the crack

Use your household cleaner to clean the area around the crack. Remove any dirt, mold, or other debris in and around the crack as much as possible. Use a sponge to scrub the area then rinse it with clean water. 

After that, dry it up with a lint-free rag or piece of cloth. 

2. Sand the area around the crack

Use the sandpaper or emery cloth to sand the area around the crack to give it a slightly rough texture. This makes it easy to add adhesives and other parts of the fiberglass tub repair kit. 

Once you’ve sanded the area to your satisfaction, brush away the dirt either with a brush or piece of clothing. 

3. Prepare and apply the adhesive

The fiberglass tub repair kit will have an adhesive or bonding agent with preparation instructions on it. Prepare it as instructed then apply it to the crack and the sanded area around it using the included brush or a paintbrush of your own. 

Make sure the adhesive covers at least an inch away from the crack for the best results. Also, the adhesive should go deep inside the crack to saturate it as much as possible. 

Wait for the adhesive to dry before the next step. 

4. Fill the crack with fiberglass filler

After the adhesive has dried up, take the fiberglass filler from the fiberglass tub repair kit then use a putty knife to apply it and fill the hairline crack on the tub with it. Remove any excess filler around the crack. 

Wait for the filler to dry up. 

5. Sand the area around the crack again

Use the sandpaper or emery cloth to sand the area around the crack until it’s level with the surrounding area then off the dirt when done. Don’t overdo it as you will only be undoing the fiberglass tub crack repair results. 

6. Coat the crack with epoxy

Read the instructions on the epoxy or polyester resin included in the fiberglass repair kit to create the required epoxy mixture. 

Apply the epoxy mixture to the crack and the surrounding area using a brush. The epoxy mixture should cover at least 2 inches away from the crack as well for the best results. 

7. Add fiberglass clothing 

Cut a piece of fiberglass clothing (found in the fiberglass repair kit) that fits the area covered by the epoxy mixture. Put the cloth over the crack before the epoxy mixture dries up. Press it firmly into the epoxy mixture then run a putty knife over it to remove any air bubbles under the cloth. 

8. Apply more epoxy mixture

Apply more of the epoxy mixture over and around the fiberglass clothing using the paintbrush. Apply a generous but reasonable coat of the epoxy mixture in this stage. 

9. Apply a sealer or finishing coat

The last procedure is to apply a sealer (also found in the fiberglass repair kit) on the cracked area. The sealer protects the layers of materials used to seal the crack from damage by water and other substances. 

Once the sealer is dry, you’re free to use your bathtub once again. You can add a buffing compound to restore the glossy finish of your fiberglass bathtub before using it again. This procedure on how to patch a crack in fiberglass tubs applies to all cracks in the bathtub including how to fix cracks in the bottom of the fiberglass bathtub and how to fix a cracked tub surround.

The video below gives a summary of this process:

Shower pans made of fiberglass can also be fixed with this procedure when they develop cracks. If the cracks in the shower pan are too deep, learn how to replace the shower pan without removing tiles.

Fiberglass tub repair kit

A fiberglass tub repair kit has the tools and chemicals you need to fix a cracked or damaged fiberglass surface. You can use it on bathtubs and other fiberglass surfaces to fix the damaged areas. 

A fiberglass tub repair kit has a fiberglass cloth, resin, brush, hardener, mixing tray, mixing stick, and a spreader. 

There are two types of fiberglass tub repairs kits as follows:

1. Surface repair kit

The surface repair kit has a polymer-based paint or tape for sealing the outer surfaces of cracks on fiberglass bathtubs. 

2. Internal repair kit

The internal repair kit has a paste or glue used to fill cracks from the inside. 

Your choice depends on the depth of the crack on your bathtub. 

What causes a fiberglass tub to crack? 

If you’re wondering why your bathtub is cracking, some of the causes of cracks in fiberglass tubs include the following:

1. Normal aging 

Normal usage of the bathtub will lead to wear and tear. The surface of the tub will slowly wear out and make the tub thin enough for cracks to develop. 

2. Insufficient support

Fiberglass bathtubs can develop cracks when placed on uneven ground since the weight of the user won’t be distributed evenly. 

3. Use of harsh cleaners

Some harsh cleaners such as bleach and acidic ones can slowly erode the surface of the fiberglass bathtub making it thinner and thus more vulnerable to cracking under pressure. 

4. Dropping and hitting it with heavy objects

In both cases, the impact of the hits will damage the surface of the bathtub and require proper fixing. With fiberglass being among the lightest types of bathtubs, it’s easy to carry it around hence a higher risk of dropping it.

The depth of the crack determines whether you should fix it by yourself or not. If it’s only on the surface, you can do it by yourself. If the crack goes all the way to the other side of the bathtub, you need a plumber for that. 

Plumbers often charge from $80 to repair bathtub cracks which is costlier than the price of fiberglass repair kits (about $50). However, plumbers know how to patch a fiberglass tub better given their skills and experience. 

Will Flex Seal fix a cracked bathtub? 

Flex Seal does not fix a cracked bathtub since it has no structural value. While Flex Seal can stop a leak, it cannot fix a crack as the crack continues growing underneath the layer of Flex Seal. 

How to prevent cracks in fiberglass tub

You can prevent cracks in tubs by doing the following:

  • Ensure the bathtub is properly supported and placed on even ground. 
  • Avoid cleaning the tub with harsh cleaners. Instead, use the recommended cleaners or even mild household cleaners. 
  • Avoid dropping the bathtub on the floor or hitting it with heavy objects. 
  • Rinse it after every use to prevent harsh chemicals from building up on the surface of the tub. 
  • If the bathtub already has a crack, drill small stop-drill holes ¼ inches deep at the ends of the cracks to stop the cracks from extending further then fix it as shown above.

Generally, protect the bathtub from any form of abuse and you’ll rarely see cracks on it. If it has developed too many cracks, have it refinished to restore its looks and integrity. 

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