How to Fix Hairline Crack in Fiberglass Tub – 8 Step Guide

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A hairline crack in a fiberglass tub is a very common occurrence.

It can occur as a result of a hit from a door or furniture falling on the tub, or someone dropping a heavy object onto the tub.

Whatever the reason, it is good to know how to fix a hairline crack in a fiberglass tub.

Fiberglass bathtubs are one of the most popular options to go for when you are getting a new bathtub installed in your bathroom.

The reasons are pretty clear. They are easy to take care of, simple to install, affordable, and available in a wide range of designs.

If you have such a bathtub in your bathroom, then you are in the right place. Here, you can learn more about the crack issues that may arise on fiberglass tubs and how to fix them. Let’s dig a bit further, shall we?

how to fix hairline crack in fiberglass tub

How to Prevent Hairline Cracks from Developing in Fiberglass Tubs

Prevention is always better than cure. Here’s how to prevent cracks in your fiberglass tub:

• Make sure your tub is adequately supported. Also, the ground it’s placed on must be even to avoid cracks.
• Be sure not to hit your bathtub with heavy objects or drop it on the floor.
• Don’t clean your bathtub with harsh cleaning products. It’s better to use mild household cleaners or recommended cleaners.
• After every use, rinse your tub thoroughly to ensure that harsh chemicals do not accumulate on its surface.

How to Fix a Hairline Crack in Fiberglass Tub

If a hairline crack has already occurred on your bathtub, you should try to fix it yourself to avoid plumbing repair costs.

This job can’t be done without the right fiberglass tub repair kit.

The cracks are likely to develop from time to time, so it pays to buy a fiberglass repair kit.

Fiberglass Bathtub Repair Kits – Selecting a Kit

Consider the material your bathtub is made of when purchasing a tub repair kit.

While some repair kits work for just about every material, others are designed specifically for fiberglass tubs.

Fiberglass bathtub repair kits contain chemicals and tools required to repair a damaged or cracked area, including:

  • Fiberglass cloth
  • Resin
  • Stirring tool
  • Mixing stick with a mixing tray
  • Thickener
  • Spreader
  • Hardener
  • Sandpaper
  • Colorants
  • Brush
  • Protective gloves

Keep in mind that your fiberglass repair kit may not have all of the items listed above, so you will have to buy them separately if needed.

It is important to pick a suitable repair kit based on the crack depth. There are 2 types of bathtub repairs kits to choose from:

Internal tub repair kit – It comes with glue for filling bathtub cracks from the inside.

Surface tub repair kit – It includes a tape or polymer coating that’s meant to seal the exterior surfaces of fiberglass tub cracks.

how to fix hairline crack in fiberglass tub

How to Fix a Hairline Crack in a Fiberglass Tub in 8 Simple Steps

Once you’ve purchased a tub repair kit, you can begin the process of repairing a hairline crack in your bathtub.

Follow the step-by-step instructions below.

Step 1: Clean & Dry the Area around the Hairline Crack

It is critical to make sure the area around the crack is perfectly clean before starting to avoid issues later on.

Clean the area around the crack using a sponge and household cleaner to remove any mold, grime, dirt, or other sediments.

After scrubbing the area, wash it away thoroughly with warm water.

Take a piece of clothing or rug and dry this area up.

Step 2: Sand the Area That Surrounds the Crack

Sand away this area with emery cloth or medium-grit sandpaper (better option) until it gets a somewhat rough texture.

That will make it easier for you to apply adhesives in the next step.

Once done, remove any dust and dirt by brushing it away with a clean piece of cloth or soft brush (better choice).

Step 3: Add the Adhesive

The bathtub repair kits usually come with a bonding agent or adhesive.

In addition to the hairline crack itself, you also need to add the adhesive about 1 inch around it with the brush (it should be included in the repair kit).

To saturate the crack adequately, the adhesive needs to go deep inside it.

Before going to the next step, leave the adhesive to dry up.

Check the instructions provided in your kit for the suggested drying time.

Step 4: Fill the Hairline Crack with the Filler

You should have a putty knife at hand for this task.

Check if there’s the filler included in the tub repair kit. If so, fill the hairline crack with it using a putty knife.

Scrape off the surface to get rid of excess fiberglass filler. This will ensure that the surrounding bathtub surface and repair area is in the same line.

Let the filler dry completely. Wait as long as the kit instructions recommend.

Step 5: Repeat Step Two, But Now with a Fine-Grit Sandpaper

Keep sanding the repair area with fine sandpaper (180-220 grit) to eliminate any dust and excess filler as well as other unwanted particles around the crack.

The goal is to get a perfectly flat area.

You have to be careful not to overdo things because you may undo the repair results.

Step 6: Take Epoxy and Coat the Bathtub Crack

Create the appropriate epoxy blend by mixing the gel coat as per the repair kit instructions.

Once it gets a good consistency, apply the mixture with a paintbrush.

Make sure the epoxy covers the surrounding area, not only the hairline crack.

A good rule of thumb is that it covers 1-2 inches around the cracked area.

Step 7: Apply Fiberglass Clothing

Cut a piece of clothing fitting the coating size and put it over the cracked area.

Make sure the gel didn’t get dry before applying fiberglass clothing.

Firmly press a piece of clothing over the epoxy mixture.

Take a putty knife and run it over the cloth to fix bubbles underneath.

The cloth should be flush with the surrounding tub surface before applying another layer of epoxy gel.

Step 8: Apply a Finishing Coat or Sealer

Applying a sealer to the crack is the last step on how to fix a hairline crack in a fiberglass tub.

The sealer is intended to protect the underlying layers from damage caused by harsh cleaners and water.

Add a coat of sealer and leave it to dry.

You may also consider applying a buffing or polishing compound, too. It can do a great job of restoring the glossy finishing and making your tub look brand new.

Once done, you can enjoy using your fiberglass bathtub and having a bath again.

This step-by-step procedure should do the trick for hairline cracks. If it doesn’t work, you may have to buy a new bathtub.

This process can also be used for repairing acrylic bathtubs.

You can use it when fixing a cracked bathtub surround or patching a crack in the bottom of your tub.

If your tub has a lot of cracks that tend to recur every now and then, you should refinish it, as it’s going to break apart after a while.

How Much Do Fiberglass Tub Repairs Cost?

Depending on the type of damage, the cost of fixing a fiberglass bathtub typically runs from $50 to $300.

Of course, it also depends on whether you hire a professional or do everything by yourself. Professional plumbers usually charge from $100 to $160/hour for hairline crack repairs.

Can I Repair My Cracked Bathtub with Flex Seal?

Flex Seal doesn’t have structural value, so it can’t fix cracked bathtubs.

While it’s capable of stopping a leak, the fiberglass cracks will continue to grow under this layer.

What Causes Hairline Cracks in a Fiberglass Tub?

The most common causes of these cracks include the use of harsh cleaners, insufficient support, and bathtub aging.

Besides, hairline cracks also develop as a result of hitting fiberglass tubs with sharp or heavy objects.


Fiberglass bathtubs are something that is both practical and aesthetic. However, because these bathtubs are made from epoxy resins, they are often susceptible to damages like hairline cracks.

That’s why you need to know how to fix a hairline crack in a fiberglass tub if you plan to install it in your bathroom.

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