How to Fix a Crack in a Fiberglass Bathtub

Cracks in a bathroom tub are not only unsightly they can also be the cause of further damage to your bathroom if not properly repaired. You will want to get a crack fixed as soon as possible to keep water from ruining the wood under and around the bathtub. A fiberglass tub repair kit makes the tub, a DIY project.

Things You'll Need

  • Fiberglass tub repair kit including:
  • Polyesther resin
  • Colorant
  • Thickening agent
  • Hardener
  • Cardboard for working surface
  • Disposable container and utensil for stirring
  • Fiberglass mesh or mat, (if crack over ¼” wide)
  • Utility knife for cutting mesh or mat materials

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Clean the area to be repaired and lightly sand it to remove any sharp edges, oils and soap scum, for good adhesion.

If the crack is over ¼”, you will need to cut a piece of the fiberglass mesh material, slightly larger than the crack. Also, keep in mind that you may need more than one layer, depending on the size of the crack.

Read the instructions of the kit to determine the measurements needed, since exact measurements are required for the mixture to set up properly, and figure out what tinting requirements you are going to need.

Get a piece of cardboard to mix the resin on, since it can be difficult to remove, if spilled. Acetone is the only thing that will get it up, and it can require quite a bit of work, once hardened. Work with disposable mixing tools and container, but not Styrofoam, because it can eat right through it.

Measure the polyesther resin and hardener, according to the proportions listed in the instructions, which are usually equal portions.

Add the needed colorant according to proportions needed to match the fixture before adding the hardener. The color should be just slightly darker, since it will dry slightly lighter.

Add the thickening agent to desired thickness and then add the hardener. If you are patching a crack on the wall of the floor, you will need it slightly thicker than a horizontal crack in the floor.

Mix the resin mixture thoroughly so that it will harden evenly. Keep in mind that you only have 10 to 15 minutes before the resin hardens, and will be unusable.

Apply the mixture, and if fiberglass mesh or mat is used, then make sure you saturate it thoroughly and add another layer of mesh or mat, if needed. You will want to apply the mixture slightly higher than the surface, since you will be sanding it smooth.

Allow the patch to dry thoroughly, usually at least 2 hours at room temperature before sanding. If using a fiberglass material, you may need to trim any edges, before sanding with coarse grit sandpaper. You can start working your way to medium, then fine sandpaper for a smooth finish.

Mix a finishing layer of the resin and colorant to overlay the first patch. This will be without adding the thickening agent, it is important to smooth this layer since this will be the final surface, other than sanding it a little with very fine sandpaper after it hardens.

Tips & Warnings

  • Some kits have different instructions than others so read through the instructions before beginning. If the crack is in the bottom of the tub, you may want to fill beneath the tub with expandable foam to keep it from recurring, and add support to the fiberglass patch. If the crack is more than ¼", you will need fiberglass mesh material for reinforcement. Keep in mind that you have to work quickly once the hardener is added. Be sure the tub is fiberglass, or the fiberglass patch kit won't work. Match the color. Most kits come with colorant for white, off white and almond. If your tub is a different shade, you will need to find colorant to get a close match.
  • Avoid spills because it is hard to clean up; if spilled, use acetone. Keep the room well-ventilated, and acetone is highly flammable.
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