Can I just caulk a cracked fiberglass tub?

No. Bathroom silicone skins over a drip and then peels. It never wets the glass fibers, so the shell is still cracked and will keep moving underfoot.

Do I need a fiberglass cloth repair kit or is epoxy enough?

A short, still crack can take epoxy in a V-groove by itself. A long split, a crack that runs to a corner, or one that opens when you stand nearby needs cloth bedded in the same epoxy so the patch spans past both ends.

How do I tell a gelcoat crack from a crack through the shell?

A gelcoat hairline stays put when you press next to it and does not show weave. A through-crack shows dull glass fibers in the groove, or the edges open when you stand in the tub. Through-cracks need a structural fill, not a surface paint.

Will epoxy hold if the tub floor feels soft?

No. A spongy floor means the wood base under the shell has rotted. Glue can patch the laminate; it cannot replace the support. Replace the unit.

Can I use the tub the same day I repair it?

No. Fill and finish both need a full cure before standing water hits them. Using the tub early lifts the patch or leaves a white, under-cured film.

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Guide

Repair a cracked fiberglass tub or shower

Use 2-part epoxy after you grind the crack to bare glass fibers. Bathroom caulk will not wet out the laminate. If the floor feels spongy, replace the unit — glue cannot rebuild a rotten base.

Fiberglass (FRP / GRP)

This task involves: Water immersion

Best Glue for This Repair

2-part marine or structural epoxy

Wets out exposed glass fibers and cures into the same kind of rigid matrix as the original laminate, so the shell can carry weight again.

Extra prep

Grind or sand the bonding area with 80-grit abrasive until glass fibers are exposed, and wipe clean with acetone.

Not for: Flexing or movement

Reviewed and adjusted by an editor.

Why 2-part marine or structural epoxy Works Here

A high-strength composite consisting of woven or chopped glass fibers set inside a cured thermoset resin. Because the cured resin is thermoset (crosslinked), it cannot be re-melted or chemically solvent-welded. Marine-grade 2-part epoxy is the gold standard for repairs, creating structural bonds that match or exceed original laminate strength. Polyurethane marine adhesives excel on high-vibration hulls and through-hull fittings.
Marine-grade 2-part epoxy wets out and cross-links with the composite resin matrix, creating structural joints stronger than the original fiberglass.
Grind or sand the bonding area with 80-grit abrasive until glass fibers are exposed, and wipe clean with acetone.

How to do it

Repair a cracked fiberglass tub or shower: Step by Step

Dust, solvent, and resin

Sanding fiberglass throws itchy glass dust. Acetone and epoxy irritate skin, eyes, and lungs. Work with a respirator, eye protection, and gloves, and keep the room vented. If you get dust or chemicals in your eyes, have trouble breathing, or feel unsure at any point, stop and get medical care.

Dry the tub completely and keep water off it. Press along the crack: a hairline in the gelcoat stays still; a through-crack shows fibers or opens when you stand nearby.
Grind or sand a shallow V along the crack with 80-grit until the glass weave is dull and bare. Wipe the dust off with acetone and let it flash dry.
Mix 2-part epoxy and work it into the V so it wets the fibers. For a long split, bed a strip of fiberglass cloth in the same epoxy and extend it past both ends of the crack.
Leave the repair still until the epoxy has fully cured, then sand the ridge flush with the surrounding shell.
Recoat the patch with gelcoat or a tub-repair finish if you want the color back. Do not fill the tub until that finish has cured.

Conditions

When to Use Marine polyurethane adhesive sealant Instead

A crack that opens when you stand in the tub is flexing. Rigid epoxy will crack again on a moving shell — switch to a marine polyurethane sealant on a seam, or back a long split with cloth-reinforced epoxy. Soft, spongy floors mean the wood base is gone; replace the tub.

Alternatives

Other Adhesives That Work

Marine polyurethane adhesive sealant
Stays rubbery under standing water and hull-style flex. Use it on a moving seam, not as a fill for a rigid floor crack.
Better when: Flexing or movement, Water immersion, Outdoor exposure

Avoid

Glues to Avoid for This Repair

Avoid
Bathroom silicone caulk
It skins over a drip for a short while, then peels. It never wets the glass fibers, so the shell is still cracked underneath.

Questions

Common Questions About This Repair

Can I just caulk a cracked fiberglass tub?: No. Bathroom silicone skins over a drip and then peels. It never wets the glass fibers, so the shell is still cracked and will keep moving underfoot.
Do I need a fiberglass cloth repair kit or is epoxy enough?: A short, still crack can take epoxy in a V-groove by itself. A long split, a crack that runs to a corner, or one that opens when you stand nearby needs cloth bedded in the same epoxy so the patch spans past both ends.
How do I tell a gelcoat crack from a crack through the shell?: A gelcoat hairline stays put when you press next to it and does not show weave. A through-crack shows dull glass fibers in the groove, or the edges open when you stand in the tub. Through-cracks need a structural fill, not a surface paint.
Will epoxy hold if the tub floor feels soft?: No. A spongy floor means the wood base under the shell has rotted. Glue can patch the laminate; it cannot replace the support. Replace the unit.
Can I use the tub the same day I repair it?: No. Fill and finish both need a full cure before standing water hits them. Using the tub early lifts the patch or leaves a white, under-cured film.