How to Fix a Crack in a Fiberglass Tub

Cracks in fiberglass tubs can be fixed by sanding the area, applying a fiberglass filler, epoxy mixture, and then adding a sealer on top. Small cracks can grow and eventually destroy the entire bathtub, so it's worth taking the time to get them repaired when they first develop.

If you enjoy DIY projects, you'll be happy to know that this is a task you can do yourself. This step-by-step guide will show you how to make the repairs and it'll provide you with some pro tips to prevent the cracking from starting in the first place.

Why Do Fiberglass Tubs Crack?

Thanks to the unique makeup of fiberglass, you can expect your fiberglass bath tub to take a beating. But that doesn't mean they're indestructible. It's not uncommon to find hairline cracks forming in your tub, here are a few reasons these annoying problems begin:

Prolonged Use - A quality-made fiberglass tub can last for up to 15-years, but after this point, it's normal for them to begin to develop hairline cracks and show signs of wear and tear.

Heavy Objects - Too much pressure and weight on a fiberglass tub can cause a small crack to form. The crack could be caused by a heavy object falling or even by your body jumping in quickly.

Harsh Cleaners - The repeated use of harsh chemical cleaners on a bathtub can slowly wear down the surface and after a few years, it can start to crack or fade.

Lack of Support - When a bathtub is installed incorrectly and without the required support underneath, it won't be able to bare weight as effectively. The constant strain on and the flex of the fiberglass will inevitably lead to cracking.

Dropping the Tub - Whether your tub was dropped before installation or dropped during a renovation, there's a good chance it'll suffer a blow and develop a crack.

How to Fix Fiberglass Tub Cracks

At the first sign of a crack in your fiberglass tub, you'll want to take action as soon as possible to get it fixed. Thankfully, it's a fairly easy DIY job that can be done with some basic materials.

There's a lot of different fiberglass products available, and one option is to purchase a fiberglass repair kit. These kits are convenient and typically save money since they include all the supplies required in a single kit. 

We recommend the products made by Bondo (which we linked to below in the guide), but whichever repair product you choose, make sure to follow their directions as they may differ from those outlined below.

Step-by-Step Guide to Fiberglass Tub Repair

Before starting, it's always a good idea to ensure there isn't any dirt, mold or grime build up in the crack. Using a gentle product suitable for fiberglass, clean the area with a sponge and water.

Be sure to rinse with clean water once you're done, and use a microfiber cloth to further clean and dry the area to ensure there isn't any moisture left in the crack.

Step One: Sand the Area

Although it's not necessary, you may want to use some masking tape to isolate exactly where you'll be working. 

Using a fine grit sandpaper, sand around the crack area with a few inches of space on each side of the crack. The goal is to create a slightly rough texture around the crack so the fiberglass resin will better adhere.

Step Two: Apply the Resin

• Mix about 4 oz of Bondo Fiberglass Resin and hardener in a mixing cup. Check the instructions on your product to ensure you get the mixture correct.
• Next, use a paint brush to apply the Bondo Resin by dabbing it to the cracked area.
• Cover the crack with a fiberglass mat and apply more resin on top. Dab the Bondo Resin onto the mat and begin brushing outwards.
• Allow the area to dry, it'll typically take about an hour.

Step Three: Apply Bondo Glass and Bondo Putty

• Apply the Bondo Glass Reinforced Filler onto the cracked area. Mix it together with the cream and fold it over with the spreader. Spread and smooth it out across the area.
• Once the Bondo Glass drys, use your sand paper to scuff it up and feather down the edges. Then clean up the area.
• Apply the Bondo Putty to the cracked area. Add the hardener and mix and smooth in the same manner as the Bondo Glass. Apply a second coat of bondo putty if you choose.
• Once the Bondo Putty has dried, sand the area until it's smooth. Wipe down the area.

Step Four: Apply Finishing Coat

• Using an appliance epoxy like the one from Rust-Oleum, you can apply the finishing coat by spraying the area. In some cases you may want to use a primer before the finishing coat.

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Pro Tips for Prevent Future Cracks

Although bathroom tub cracks can be fixed fairly easily, it's not a job you want to do too many times. To prevent your fiberglass tub from being damaged prematurely, here's a few tips to keep it crack-free.

Proper Installation - Whether you're installing a new tub or moving an old one, you should always ensure that it has adequate support underneath. Fiberglass is a flexible material that needs a firm foundation beneath it, if not, you'll start noticing a "mushy" feeling and cracks will begin to form.

Gentle Cleaners - Fiberglass requires a gentle cleaning product since harsher products can wear down the surface. As a general rule, you should never use a product that contains bleach.

React Quickly to Cracks - A hairline crack can easily develop into a larger one, and in some cases even split your bathtub in half! As soon as you notice a crack, take steps to fix it. It's also a good idea to inspect the bathtub's surface when cleaning so you can check for signs of damage.

Invest in a Repair Kit - If you own a fiberglass tub, it's a good idea to invest in a fiberglass repair kit so you have it on hand when you notice a crack. These kits typically have the tools you'll need to repair minor cracks.

Repair or Replace

Fiberglass tubs are a great choice for bathrooms, but if yours has a crack you may be trying to decide if you should repair it or simply purchase a new one.

If you enjoy DIY projects, there's no reason why you can't roll up your sleeves and do the work yourself, or you may want to hire someone to come in and fix it for you. Either way, it'll most likely be less expensive than purchasing a new tub.

However, if you're considering remodeling your bathroom, a cracked bathtub may to the perfect excuse to get started. A new tub is a nice addition to a bathroom remodel and there's nothing quite as exciting as a new bathroom!