repair cracks in a fiberglass tub: step-by-step tutorial3 min read

Fiberglass tubs are cheap and relatively easy to maintain, making them the preferred choice for many homeowners. But as attractive as they are, fiberglass tubs can easily develop cracks and chips that compromise their structural integrity. So, how do you repair cracks in a fiberglass tub?

Drain the tub to expose the crack and clean the damaged area. Cut a suitable piece of fiberglass cloth to seal larger cracks and holes. Mix the repair material and apply it quickly. Add more resin and fiberglass cloth as needed. Sand down the patch, then buff and polish the repaired area.

Leaving cracks unattended can cause irreparable damage to your fiberglass tub, especially if they increase in size. So, to extend the life of your tub, it’s best to grab a commercial repair kit and get to work. Let’s go through the details of how to do it properly.

Fiberglass Tub Cracks: Understanding the Root Causes

While fiberglass composite materials are quite strong, they can suffer damage and develop spider, hairline, and stress cracks. These are the main reasons for a fiberglass tub developing cracks:

Abrasive Cleaners Can Weaken the Finish

Let’s face it — our bathtubs can get pretty gross over time, and a quick wipe-down won’t remove some of the tougher stains. In that case, you or whoever was charged with cleaning the tub may have been tempted to break out the big guns: bleach, heavy-duty commercial cleaners, and ammonia.

These chemicals can get rid of stubborn stains, but they also wear down your tub’s finish. Eventually, cracks may begin to form, and some parts of the tub’s surface may even chip.

Impact From Heavy Objects

It’s not uncommon to have beautiful objects around your tub area, whether it’s a small statuette or a heavy vase full of flowers. If any of these objects happen to fall into the tub, they might cause a crack or dent in the finish.

A Poorly Designed Supporting Bed

A fiberglass tub should have proper support during installation. Otherwise, the combined weight of the water and your body can press against the weakly supported parts of the shell and put a strain on it.

The resulting cracks do more than just take a toll on the aesthetics of your tub. If you don’t repair them soon enough, they can spread throughout the tub, allowing water to seep through and damage the ceiling underneath it.

Is Fiberglass Harder to Repair Than Other Materials?

Fiberglass is a lot easier to repair than many other materials like steel and aluminum. That’s because there’s no extended repair process involving disassembly, structural repairs, priming, reassembly, and detailing. Often, it’s just a matter of you applying more fiberglass mesh and resin to the damaged areas.

Repair a Crack in a Fiberglass Tub: 9 easy steps

You need a fiberglass repair kit (on Amazon) to fix the crack in your tub. Most kits come with all the tools and materials necessary for the job, but you can also buy the items separately.

Here’s what you can expect to find in your repair kit:

  • Fiberglass mesh
  • Polyester resin
  • Hardener
  • Thickener
  • Tinting products (colorants)
  • Sandpaper
  • A mixing container and stirring tool
  • Protective gloves that are resistant to the chemicals

The Repair Process

Follow these steps to repair the cracks in your fiberglass tub:

  1. Drain the Tub to Expose the Damage: Turn off the power to your fiberglass tub and drain it to expose the crack.
  1. Wear Protective Clothing: Don protective gloves and safety goggles to protect your hands and eyes. You should also wear a respirator to avoid inhaling fiberglass dust and any of the resin fumes.
  1. Clean the Area to Be Repaired: Cut any jagged and protruding fiberglass material around the cracked area, then sand down the area lightly. Rinse the surface with acetone to ensure proper adhesion of the repair product.
  1. Cut a Suitable Patch of Fiberglass Cloth: If the crack is more than 1/4-inch wide (50 mm) or has an open hole that the resin mixture won’t fill, cut a piece of fiberglass cloth larger than the hole. If the crack is too wide, you’ll need multiple layers of fiberglass cloth to get good results.
  1. Mix the Fiberglass Repair Chemicals: Read the instructions that came with your repair kit carefully before you proceed. Place a protective material like cardboard on the surface where you’ll place the mixing container.

Carefully measure and pour a suitable amount of polyester resin into the mixing container and add a proportionate amount of colorant. Mix the resin-colorant mixture to a consistent thickness. Add the hardener according to the kit’s instructions.

As a good estimate, add five drops of hardener for each tablespoon of the resin-colorant mixture. Too much hardener will accelerate the setting process, giving you very little working time. And if you add too little, the resin won’t set properly and will become tacky.

Once you’ve added the hardener, mix the material thoroughly and apply it as soon as possible, within 10-15 minutes. Otherwise, the resin will become unusable.

  1. Apply the Repair Material to the Damaged Area: Use a flat tool like a putty knife to scoop a reasonable mixture of the repair material and apply it to the cracked area. For large cracks and holes, place the fiberglass cloth over the damaged area and press it into the resin mixture.

Apply the resin mixture evenly and to a level slightly higher than the original surface so you can easily sand it down and feather it smooth when finished. Wait for about two hours for the repair material to harden.

  1. Sand Down the Patch: Trim any protruding fiberglass cloth with a sharp utility knife if you applied it to the patch. Next, sand down the patch carefully so you don’t scuff adjacent areas. Sand with coarse, medium, and fine grit sandpaper in that order for a smooth finish.
  1. Overlay the Patch With More Resin & Colorant: For the best results, mix another batch of resin and colorant to build up the repair further. Don’t thicken it.

    Overlay the first patch with a small paintbrush, smoothening it out as much as you can. Let the patch harden and sand it down with fine-grit sandpaper. 
  1. Buff and Polish the Repaired Surface: Apply the buffing compound that came with your kit to the repaired surface then give it a nice polish. Clean up the area when you’re done.

Ways to Prevent Cracking Your Tub

A fiberglass bathtub is generally durable and should last for years if properly maintained. Follow these tips to prevent cracking your fiberglass tub and boost its longevity:

  • While installing a new tub, ensure it has proper supports to bear additional weight during use so the material won’t strain.
  • Use cleaning agents that are safe for use on a fiberglass tub rather than harsh bleaching chemicals and cleaners with ammonia. Moreover, avoid abrasive scrubbers.
  • Be careful while handling objects in and around the bathtub. Do not throw or drop heavy objects into the tub.
  • Keep heavy decorative objects on a shelf in the bathroom, not the ledge by the tub.
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Hi, I'm Ed, and I run BuildFanatic! I enjoy providing the best possible information on a range of home improvement topics.

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