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Cracked Bathtub? How to Seal Hairline Cracks With a Repair Kit

A hairline crack lets water into the layers under a tub and then spreads. Stop it, seal it with an epoxy kit, and fix the flex that caused it.

By Jamilur Rahman Updated November 5, 2026 · 7 min read

Time
1 hour, plus curing
Cost
$20-$50
Difficulty
Medium
A thin hairline crack running across the white floor of a bathtub near the drain
In this guide
  1. Quick answer
  2. Free vs. permanent fix
  3. What you'll need
  4. Why it happens
  5. Try this first (free)
  6. When free fixes aren't enough
  7. Buy the fix
  8. What is your tub made of?
  9. Work safely with epoxy
  10. What to skip
  11. FAQ
  12. Related problems

Quick answer

To fix a bathtub crack that is only a hairline, clean and dry it, stop it spreading, then fill it with a tub and tile epoxy repair kit. Most cracks in acrylic and fiberglass tubs come from a floor that flexes or from a hard knock. A kit seals the crack, but a tub floor that still flexes will crack again, so check that too.

Free fix vs. permanent fix

Free fix · 20 minutes

Stop it spreading and keep it dry

  • Check the ceiling or access panel below for leak stains
  • Press the tub floor to see if it flexes under your hand
  • Drill a tiny stop hole at each end of the crack on acrylic or fiberglass
  • Wipe the tub dry after each use until you seal it

How long it lasts: Slows the crack and keeps water out for days. It does not seal the crack, so it will still let water into the layers under the tub.

Most readers choose Permanent fix
Tub and tile repair kit (epoxy) (illustration)
Illustration

A tub and tile epoxy repair kit

Epoxy fills the crack, bonds to the tub and sets hard. It seals out water so the layers under the surface stay dry, and you can sand and polish it flush. A kit has the filler, hardener and sanding pad in one box.

What to get: Tub and tile repair kit (epoxy)

How long it lasts: Years when the prep is thorough and the tub no longer flexes.

Check price on Amazon

As an Amazon Associate I earn from qualifying purchases.

What you'll need

  • Rubbing alcohol and a cloth

    For cleaning the crack before you fill it

    You have it

  • Drill with a small bit

    A bit about 1/16 inch wide, for the stop holes

    You have it
  • Wet and dry sandpaper (illustration)
    Illustration

    Wet and dry sandpaper

    Coarse for opening the crack, fine for polishing the repair

    Check price
  • Our pick
    Tub and tile repair kit (illustration)
    Illustration

    Tub and tile repair kit

    Choose one made for your tub material, in white or a tint that matches

    Check price
  • Low-expansion spray foam (illustration)
    Illustration

    Low-expansion spray foam

    Only if the floor of the tub flexes and you can reach the gap under it

    Check price

Why it happens

1

A tub floor that flexes

Acrylic and fiberglass tubs rely on support underneath. If there is a gap between the tub and the subfloor, the floor flexes every time you step in, and a crack forms at the stress point.

2

A hard knock or a dropped object

A heavy bottle, a dropped tool or a ladder foot can crack the surface. The damage may start as a star or a short line that grows over weeks.

3

Heat and cold swings

Very hot water in a cold tub makes the surface expand faster than the layers below. Over time that stress shows up as a hairline crack.

4

Water that reached the layers underneath

Once water gets into a crack, it swells the fiberglass layer and pushes the crack wider. A hairline can open into a long line without anyone noticing.

Try this first (free)

Do these in order, starting with the simplest.

  1. 1. Check for leaks and flex

    Look under the tub, from a basement ceiling or an access panel, for damp patches or stains. Then press on the floor of the tub with the heel of your hand. If it gives or crackles, the floor lacks support.

  2. 2. Clean and dry the crack

    Tool used: Rubbing alcohol and a cloth

    Wipe the crack and about an inch around it with rubbing alcohol. Let it dry fully, or use a hair dryer on low. Epoxy will not bond to a damp or soapy surface.

  3. 3. Stop the crack from spreading

    Tool used: Drill with a small bit

    On acrylic or fiberglass, drill a tiny hole about 1/16 inch wide at each end of the crack, through the surface layer only. This relieves the stress at the tip. Do not drill an enameled steel or cast iron tub.

  4. 4. Open the crack and fill it

    Tool used: Tub and tile repair kit

    Sand a shallow V groove along the crack with coarse paper, wipe it clean, then mix the epoxy as the kit says. Press it into the crack with the spatula, fill slightly proud and level it. Add the tint as the kit directs.

    Tub and tile repair kit (illustration)
    Illustration

    Tool for this step

    Tub and tile repair kit · Choose one made for your tub material, in white or a tint that matches

    Check price on Amazon

  5. 5. Cure, sand and polish

    Tool used: Wet and dry sandpaper

    Leave it for the time the kit lists, often a day, and keep water off it. Then wet-sand with fine paper until flush, and polish. Do not use the tub until the kit says it is safe.

    Wet and dry sandpaper (illustration)
    Illustration

    Tool for this step

    Wet and dry sandpaper · Coarse for opening the crack, fine for polishing the repair

    Check price on Amazon

When free fixes aren't enough

Epoxy seals a crack, but it will not stop a floor that flexes. If the tub floor gives under your hand, or the crack comes back near the same spot, the tub needs support under it. Low-expansion foam through an access gap can help, or a pro can add a mortar bed. If the crack is wider than a hairline, runs the length of the tub, leaks into the ceiling below, or the tub is soft and spongy, stop using the tub and call a plumber or a pro.

Buy the fix

Still stuck after the free fixes? These are the products we would look at.

Our picks compared
PickProductFixesWho it's forBuy
Best pick
Tub and tile repair kit (epoxy) (illustration)
Illustration
Tub and tile repair kit (epoxy)
hairline cracks and small chips that let water into the tub Anyone with a hairline crack or a small chip in an acrylic, fiberglass or porcelain tub. Check price
Upgrade pick
Low-expansion spray foam for tub support (illustration)
Illustration
Low-expansion spray foam for tub support
a tub floor that flexes and keeps cracking Anyone who can reach the gap under the tub, through an access panel or from below, and whose floor flexes. Check price
Best pick
Tub and tile repair kit (epoxy) (illustration)
Illustration

Tub and tile repair kit (epoxy)

Fixes: hairline cracks and small chips that let water into the tub

Look for: Made for your tub material, such as acrylic, fiberglass or porcelain; White or a tint that matches your tub color; Two-part epoxy with filler, a spatula and sanding pads; A clear cure time on the label.

Who it's for
Anyone with a hairline crack or a small chip in an acrylic, fiberglass or porcelain tub.
Honest downside
The tub floor flexes a lot or the crack is wide. Fix the support first, or call a pro.
Check price on Amazon
Upgrade pick
Low-expansion spray foam for tub support (illustration)
Illustration

Low-expansion spray foam for tub support

Fixes: a tub floor that flexes and keeps cracking

Look for: A low-expansion foam made for gaps and voids; A thin straw or nozzle for reaching under the tub; A foam that stays flexible when cured.

Who it's for
Anyone who can reach the gap under the tub, through an access panel or from below, and whose floor flexes.
Honest downside
You cannot see under the tub, or it is a heavy cast iron tub. Expanding foam can lift a tub, so ask a pro.
Check price on Amazon

As an Amazon Associate I earn from qualifying purchases. We don't show prices because they change, so use the button to see today's price.

How we picked: How we picked: this page is based on bathtub repair guides, instructions from epoxy repair kit makers, and the problems and fixes people report in owner reviews. We do not test products by hand.

What is your tub made of?

  • Acrylic or fiberglass: light, slightly warm to the touch, and a magnet will not stick. Cracks are common and repairable with epoxy.
  • Enameled steel or cast iron: cold, heavy, and a magnet sticks. These chip rather than crack. Use a kit for chips, and do not drill.

Work safely with epoxy

Open a window or run the fan, wear gloves and keep the mixed epoxy off your skin. Read the kit label for fumes, and keep children and pets out until it has cured.

What to skip

Caulk, silicone, super glue and paint

They stay soft or turn brittle, then peel away and trap water in the crack. A flexing tub pushes them out. Epoxy bonds and cures hard. Skip painting over a crack too, since the paint cracks with it.

Frequently asked questions

Can a cracked bathtub be repaired?
A hairline crack in an acrylic, fiberglass or porcelain tub usually can. Clean it, stop it from spreading, fill it with epoxy and sand it smooth. Wide cracks, cracks that leak below or a tub with a soft floor need a pro.
Why does my bathtub keep cracking?
Almost always the floor of the tub flexes because it is not supported. Each time you step in, the same stress point cracks. Seal the crack, then add support under the tub, or the repair will crack again.
How can I tell if a cracked tub is leaking?
Check the ceiling or an access panel below for stains, damp or a musty smell. You can also fill the tub a few inches, mark the water level and see whether it drops.
How long does a bathtub crack repair last?
With good prep and a supported floor, it can last for years. If it fails within weeks, the cause is usually damp prep or a floor that still flexes.

As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure

Tub and tile repair kit (epoxy) (illustration)
Illustration

Our pick: Tub and tile repair kit (epoxy)

Check price