How To Fix Hairline Crack In Fiberglass Tub: 7 Easy Steps

Fiberglass bathtubs are affordable alternatives to the more expensive ceramic ones. The material is resilient and resistant. However, if your tub is subject to heavy use or if it doesn’t have adequate support, it could develop hairline cracks. If left untreated, these cracks can develop into larger openings that are harder – and sometimes impossible – to fix.

You can fix a hairline crack in a fiberglass tub easily with a fiberglass repair kit. You’ll have to clean and sand the area first, but make sure the tub is clean of dust and completely dry, or the epoxy compound might not adhere to the surface.

What Causes A Fiberglass Tub To Crack: 5 Common Reasons

Fiberglass bathtubs are strong and resilient. They are lighter than ceramic tubs and resist longer than wooden ones. Moreover, fiberglass looks better than acrylic. However, there are things that can damage the fiberglass structure. 

Improper Handling 

Fiberglass tubs are flexible and can bend easily as they’re handled. Improper handling during transport or installation is one of the most common causes of hairline cracks. While the surface cracks during handling, the damage is often invisible to the naked eye in this stage.

As you use the tub, the cracks will get larger, and you’ll eventually be able to see them. If the tub doesn’t crack due to improper handling, the material could weaken and lead to cracking in time.

Lack of Support

Of all tub types, acrylic and fiberglass are the ones that need the most support. Due to the flexible nature of the material, a lack of support will cause the material to bend. The way it absorbs the shocks is also different when support is lacking, leading to cracking.

Abrasive Cleaners

One of the major enemies of fiberglass bathtubs is hidden in your cleaning supplies. Most people like to use abrasive cleaners to remove grime from the tub surface and maintain it like new for longer. However, these harsh chemicals can scratch and damage the tub’s topcoat.

Without the protective finish, the fiberglass degrades and becomes brittle. The first thing to notice is a change in texture and the discoloration of the affected area. In time, you can safely assume that the surface will crack.

Heavy Objects 

It doesn’t take a rocket scientist to understand that dropping heavy objects in the tub will eventually damage the material.

This includes constantly dropping shampoo or shower gel bottles, heavier plastic toys, or stick-on caddies with poor-quality suction cups that constantly fall from the wall.

Exposure to Sunlight 

Another hidden danger that you might not think about is sunlight. With the tub being located inside the bathroom, you may believe that you can rule out UV damage. However, if the tub is located under a south-facing window and you like to keep the shades up or curtains open during the day, the fiberglass surface could be exposed to plenty of UV light. 

Like other plastic materials, fiberglass isn’t immune to UV damage, the light causing the material to become brittle. 

Pair sunlight exposure with a lack of support, heavy use, harsh cleaners, or constantly dropping objects, and your tub may develop hairline cracks in the blink of an eye.

Before You Start Repairing Cracks In Fiberglass Tubs

Repairing fiberglass tubs with a fiberglass repair kit isn’t hard, but if you don’t do it well, you could risk more damage. Here are the things you need before getting started. 

Things You Will Need To Patch The Tub

  • Fiberglass repair kit consisting of polymer or epoxy resin and hardener 
  • Mixing stick
  • Paintbrush 
  • Coarse sandpaper 
  • Fine-grit sandpaper 
  • Rotary tool
  • Paintbrushes 
  • Non-abrasive household cleaner
  • Rubbing alcohol or acetone
  • Microfiber cloth
  • Rags
  • Tub paint
  • Sponge 
  • Rubber gloves
  • Mask with respirator
  • Drill (optional)
  • Two-component foam (optional)
  • Removable market

Estimated Repair Cost

Repairing hairline cracks in a fiberglass tub can cost you anywhere between $20 and $250, depending on how extensive the damage is and whether you decide to repair the tub yourself or hire a professional for the job.

Generally, fixing hairline cracks is an easy DIY project suitable for all skill levels. However, you should set aside a full day for the project. Thus, if you don’t have the time, you might want to hire someone to repair the tub for you.

Fixing Hairline Crack In Fiberglass Tub In 7 Easy Steps

Hairline cracks in fiberglass are easy to fix with a fiberglass repair kit. There are many different solutions on the market, including one-part and two-part kits. Two-part kits are your best choice, providing a more durable finish. Thus, this guide focuses on repairing the cracks with a two-part kit.

Step 1 – Assess the damage extent

Before getting started, inspect the entire surface of the tub, including its bottom and walls. Identify all cracks and dents you have to fix. To make things easier, you can use a removable marker to mark the position of each opening – you’ll have to remove the market before applying the epoxy or resin, though.

Step 2 – Fix the lack of support (if needed)

If your bathtub cracked because of inadequate support, you should fix this issue before fixing the cracks. There are two ways to do this. 

The longest and hardest way is to remove the tub and pour an adequate concrete foundation under it or build a wood support.

The quickest way is to drill a few small holes at the bottom of the tub’s front wall (as close as possible to the floor), then fill this space with two-component foam.

Inject the foam into the holes. This type of foam usually expands and hardens when it dries, providing a solid foundation for your tub.

Step 3 – Clean the tub and prepare the area 

Epoxy or resin fillers can fix smaller and larger cracks alike, but the material won’t adhere to a dirty surface. Thus, it is crucial to clean the tub properly before proceeding.

Start with a generic household cleaner. Apply the product to a soft sponge and scrub the interior of your tub.

Rinse with clean water and dry the area with a towel. Soak a clean rag into rubbing alcohol or acetone and wipe the surface to remove any oils or soap scum. Let it air dry.

Step 4 – Sand the surface 

Use 80-grit sandpaper to sand the area above the crack. The purpose is to create a concave area to fill with epoxy or resin mixture. A rotary tool can help speed up the process, but you can also sand by hand if you don’t have and don’t want to buy or rent the tool.

Wipe the dust away when you’re done, then switch to fine-grit (220-grit) sandpaper to smoothen the surface. 

Wipe away the dust, then clean with a damp cloth and pat dry with a towel. Let the surface dry completely before proceeding. 

Step 5 – Mix the fiberglass repair compound

Commercial fiberglass repair kits consist of epoxy or resin mix and a hardening agent. The two must be mixed together right before applying them. The hardening agent hardens fast, so you’ll have to be quick at applying the mixture to all damaged areas.

Simply mix the two parts of the kit as instructed on the product’s label and stir with a mixing stick.

Step 6 – Repair the cracks 

Use the mixing stick to apply the filler on the cracks, then use the spatula or scraper included in the kit to spread it over the damaged area. Apply some pressure to force the filler into the crack. 

Let it dry as instructed, generally for several hours, then use the scraper to remove any excess material. 

Step 7 – Refinish the tub

Most two-part fiberglass fillers come in various colors that match the most common fiberglass tub shades. However, if your tub has an unusual color, you may want to use white filler and paint it later with fiberglass paint.

Once the filler is dry and set, sand the excess with 220-grit sandpaper until it’s flush with the rest of the tub.

Apply a thin layer of paint in the desired color and let it dry, then apply a second layer. Once dry, apply a protective layer of topcoat on the entire tub to help the patched area blend in. Let it dry; then, you can enjoy your tub.

Will Flex Seal Fix A Cracked Bathtub?

You could use Flex Seal on your fiberglass tub as a temporary fix. For instance, to prevent a crack from getting worse until you get to the store and buy a fiberglass filler. However, Flex Seal doesn’t fill the crack, so the damage will keep getting worse under the adhesive. The only way to repair fiberglass properly is with a specific fiberglass filler.

How to Remove Flex Seal from Fiberglass?

You can remove Flex Seal from fiberglass with acetone or rubbing alcohol. Soak a rag in solvent and scrub the surface until all sealant is removed. 

6 Tips To Prevent Hairline Crack In Fiberglass Tub

Hairline cracks are easy to fix, but they often indicate a more serious issue. Here are a few tips to prevent cracks from reemerging. 

Make sure the tub is handled correctly

Whether you arrange for your own tub transport or have it delivered, try to pay as much attention as possible to how the fixture is handled. Make sure the transport company loads and unloads it carefully, without dropping the box on the pavement. Remember that even the smallest shocks can cause the flexible material to crack.

Inspect the tub before installing it

Even if you’ve paid attention to the transport, inspect the tub before installing it. Moreover, be careful how you or the contractor handles the tub during installation. 

Provide adequate support 

A lack of support is one of the main causes of hairline cracks. Provide a solid foundation for your tub with a plywood frame or concrete blocks. The main areas to focus on are the ends and center of the tub. The ends are usually curved and need lateral support, whereas the center is the most solicited area.

Keep an eye on your kids while they’re bathing 

Right after a lack of support, physical damage is another frequent cause for cracks and dents. While anyone can drop a shampoo bottle, for example, kids find it particularly entertaining to throw toys in the tub and watch them splash the water around. 

To prevent tub damage, make sure all bath toys are made of soft materials, such as rubber or silicone. Avoid using heavy, plastic toys in the bathroom, including plastic buckets and heavy water guns. 

Fasten the caddies to the wall

Bathroom caddies provide much-needed storage space, but they can damage your tub if they aren’t fastened securely to the wall. Steer clear of stick-on caddies with suction cups, as most suction cups are not strong enough to withstand the weight of your bath products. Opt for wall-mounted or freestanding caddies instead.

Protect the tub from sun rays 

If you have a sunny bathroom and notice that sunlight falls directly on your tub for most of the day, consider using the blinds, a thicker curtain, or cover your tub with a towel when not in use. In this way, you can prevent UV damage.

Frequently Asked Questions

Will a hairline crack in the tub leak?

Hairline cracks rarely leak, but they could develop into larger cracks or openings that could leak. Sometimes, these openings happen under the tub’s finish and may be hidden from sight; thus, your tub could leak without you noticing it.

If left untreated, these cracks could lead to constant leaking each time you use the tub. Consequently, you may start having mold and mildew issues in the bathroom, not to mention damaged floors, subfloors, and floor joists.

How do I know if my tub is fiberglass?

Figuring out whether your tub is fiberglass or acrylic can be challenging, but there are telltales. As a general rule, acrylic tubs have thinner edges and are warmer to the touch. They are also available in more colors – most fiberglass tubs are white, so if your fixture is ivory, cream, black, or any other color, it’s likely acrylic.

Fiberglass also has a rougher texture, whereas acrylic is smoother. Moreover, acrylic often feels more flexible than fiberglass.


Fixing hairline cracks in a fiberglass tub is child’s play if you have the right tools and materials, but they often indicate an underlying problem. If you don’t fix that, rest assured those cracks will reemerge.

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