How to Fix a Hairline Crack in a Toilet Bowl or Tank


Cracks in a toilet bowl or tank can be annoying and lead to leaks or other problems if not addressed. Hairline cracks may seem minor, but they should be repaired as soon as possible before they turn into larger issues. The good news is that hairline cracks in a toilet can often be fixed with some simple supplies and DIY methods.

In this comprehensive guide, we’ll cover everything you need to know about identifying, preparing for, and repairing hairline cracks in a toilet bowl or tank. With a few tools and steps, you can permanently seal up small cracks and prevent further damage.

What Causes Hairline Cracks in Toilet Bowls and Tanks?

There are several potential causes of hairline cracks in toilet bowls and tanks:

  • Age and wear – Over time with repeated use, the porcelain material can develop stress fractures and small cracks from the expansion and contraction. Older toilets are more prone to cracking.
  • Impact damage – Dropping heavy objects on the toilet bowl or tank can cause immediate cracks or lead to hairline fractures that show up later. This includes impacts from cleaning tools like plungers.
  • Chemical damage – Harsh chemical cleaners, especially acidic ones, can eat away at the enamel glaze over time leading to cracking. Disinfectant tablets in the tank can also damage the porcelain.
  • Improper installation – If the toilet is not set down gently and evenly on the floor wax seal, it can cause stress that leads to hairline cracks appearing.
  • Freezing temperatures – In cold weather, water left sitting in the tank or bowl can expand as it freezes and put pressure on the porcelain material. This stress can cause hairline cracks.

Inspecting and Identifying Hairline Cracks

Start by doing a thorough visual inspection of the entire toilet bowl, tank, and base. Look for any thin crack lines in the porcelain.

Here are some tips for identifying hairline toilet cracks:

  • Drain all the water out of the tank and bowl so the surfaces are dry and easier to inspect.
  • Shine a bright flashlight at an angle along the surface to highlight any thin crack lines.
  • Flush a dye tablet or a few drops of food coloring into the tank water to dye it and make cracks visible when the colored water seeps out.
  • Rub toilet wax or petroleum jelly along suspected cracks. The wax will seep into any hairline fractures present.
  • Use a magnifying glass to inspect very thin crack lines that are hard to see.
  • Check along the toilet base, tank and underside for leakage drips or stains which may indicate cracks are present.

Pay special attention around the toilet bowl drain at the bottom, around bolt holes, and any high stress points that see more pressure. Even very thin hairline cracks should be repaired before they expand.

How to Fix a Hairline Crack in a Toilet Bowl or Tank

Once you’ve identified any hairline cracks that need repairing, follow these steps to permanently seal them:

Supplies Needed

  • Sandpaper
  • Primer/cleaner
  • Epoxy adhesive sealant
  • Applicator sticks/gloves

Steps to Fix Cracks

  1. Clean and dry the area – Use a cloth to wipe dirt away from the crack. Let the area dry fully.
  2. Sand and rough up the surface – Use 220 grit sandpaper to scuff up the glazed porcelain around the crack. This helps the epoxy adhere better.
  3. Apply primer/cleaner – Wipe primer or surface cleaner over the area to prep it for the epoxy.
  4. Mix and prepare epoxy – Mix up the two-part epoxy adhesive according to package directions. Work quickly once mixed.
  5. Fill in crack with epoxy – Use an applicator stick to fill in the hairline crack with the epoxy adhesive paste. Apply a little extra over the top.
  6. Smooth the epoxy sealant – Before it dries, use a plastic scraper or putty knife to smooth out the epoxy seam for a flush finish.
  7. Allow to fully cure – Let the epoxy cure fully, which usually takes 8-12 hours. Don’t use the toilet during this time.
  8. Sand and finish – Once fully hardened, you can use fine grit sandpaper to smooth out the repair if needed.

The epoxy adhesive will permanently bond to the porcelain and seal up any hairline cracks. Be sure to follow the product directions closely. Some epoxies designed for porcelain fixtures work best. With the right steps, you can repair hairline toilet cracks and prevent further damage.

FAQs About Fixing Hairline Toilet Cracks

Can you use super glue instead of epoxy?

No, super glues like Krazy Glue are not recommended. They do not bond as strongly to the slick porcelain surface. Specialty porcelain repair epoxy is formulated to adhere to the glazed surface and flex with temperature changes.

What about using caulk to seal cracks?

Caulk alone is not strong enough to permanently seal hairline cracks, but you can apply caulk over the top of the epoxy for additional sealing if needed. Make sure to use a flexible and waterproof caulk designed for bathrooms.

How can I prevent hairline cracking in the future?

Avoid harsh chemical cleaners which can erode the glaze over time. Also install a toilet seat with buffer pads to prevent impact fractures. Make sure to set the toilet down evenly when replacing the wax seal. Also maintain warm temperatures to avoid water freezing and expansions.

Is it OK to still use the toilet with a hairline crack before repairing?

For minor hairline cracks with minimal leakage, you can continue use cautiously in the short term. But repair the crack as soon as possible to prevent larger problems. Seal any leaks to avoid damage to the surrounding floor.

How do I prep the area and remove glaze before applying epoxy?

Use 220 grit sandpaper to lightly scuff up the glazed surface around the crack. This etches the surface so the epoxy can bond. Avoid sanding too aggressively or it may damage the toilet surface further. Clean the sanded area with primer/cleaner before applying epoxy.

What do I do if the epoxy crack repair doesn’t hold?

Make sure to fully clean and dry the surface before applying epoxy. Roughening the glaze with sanding is also key for adhesion. If it still won’t bond, you may need a different epoxy product designed specifically for slick porcelain. Failing repairs usually indicate a bonding issue.


Dealing with a hairline crack in a toilet bowl or tank can seem like a nuisance at first. But left unaddressed, the cracks can lead to leaks, water damage, and even porcelain fractures down the line. With this guide, you now have the knowledge to identify small hairline cracks and the DIY steps to permanently seal them up. Repairing minor toilet cracks as soon as they appear will save you headaches and prevent expensive plumbing repairs later on. The key is using the right sanding techniques and waterproof epoxy adhesives formulated to bond with porcelain. With a simple kit of supplies and following the steps, you can become a toilet crack repair pro.