Fiberglass is a common manufacturing material that is lightweight and long lasting. Fiberglass is used to make a wide variety of items that are found around the house and outside of the house. Homeowners install fiberglass tub and shower surrounds, own fiberglass hull boats, surfboards, patio furniture, fences, sheds or recreational vehicles along with hundreds of other items. Over time, exposure to weather conditions, freeze thaw patterns, age and impact damage fiberglass can crack. Fortunately the cracks can be repaired and the bathtub enclosure, boat or RV can be restored and made useable.

Fiberglass offers homeowners and buyers a strong and durable surface that is waterproof and easy to maintain. Cracks in fiberglass can range from small spidering cracks to larger single line cracks. If water enters a narrow crack and freezes the ice expands and enlarges the cracks. Cracks left unrepaired allow water and moisture to seep under the fiberglass and destroy the substrate. Water behind a fiberglass tub or shower surround gives mold and mildew a perfect place to flourish. The dampness can also cause the wood studs behind the walls to develop wood rot. Wood rot softens the wood and takes away its strength thereby threatening the structural integrity of walls, house, boats and RV’s.

Preparing Fiberglass for Repair

Add a squirt of liquid, grease fighting dish washing soap to a bucket of warm water.

Stir the soap and water to blend it thoroughly.

Submerge a soft bristle nylon scrub brush into the soapy water, shake off the excess water and scrub the surface of the cracks using care not to wet the crack excessively.

Wet a rag with plain water and wipe the fiberglass to remove soap residue.

Dry the surface with a microfiber or chamois cloth.

Allow the fiberglass to air dry for an hour or two.

If the fiberglass surface remains dirty, you may have to take on more intensive and aggressive cleaning measures.

Repairing Fiberglass

Wrap a piece of 320-grit sandpaper around a hard rubber sanding block.

Lightly sand the surface of the cracks until the shiny surface has been removed from the fiberglass. Sanding also removes excess grease, oils and soap that were not removed by cleaning.

Wipe the fiberglass surface with a tack rag to remove sanding dust along with tiny particles of fiberglass.

Choose a matching color marine grade epoxy repair putty from a boating supply store.

Mix equal amounts of two part marine grade epoxy repair putty on a disposable plastic, plate, bowl or cup with a small wood stick.

Scoop up enough epoxy repair putty to fill the cracks on a flexible putty knife. Work on one crack at a time.

Hold the putty knife at a low angle to the fiberglass with the edge of the knife touching the fiberglass. Pull the putty knife across the crack while pushing the epoxy into the gap.

Move to the next crack and pull the knife across the crack to distribute more epoxy putty into the crack.

Let the repair putty harden and set for 30 to 45 minutes or as directed by the epoxy manufacturer’s instructions.

Wrap 320-grit sandpaper around the hard rubber sanding block and lightly sand the surface of the repair to make the epoxy putty flush with the surrounding fiberglass surface.

Wipe the surface with a tack rag to remove the sanding dust.

Wrap 600-grit sandpaper around the sanding block and gently sand the surface of the epoxy patch.

Wipe away the sanding dust with a tack rag.

Examine the fiberglass surface to ensure you have filled in all of the cracks. If you find any cracks you may have missed with the first epoxy application, mix new epoxy and spread it over the crack while forcing it into the gaps. Let the epoxy dry for 30 to 45 minutes, sand with 320-grit sandpaper then with 600-grit sandpaper.

If the all cracks have been filled with repair putty, wrap 1000-grit sandpaper around the sanding block and sand the epoxy surface making small circular motions.

Wipe the surface with a tack rag.

 Apply a coat of marine grade fiberglass wax to the fiberglass surface, covering the repairs and the entire wall, side of boat, RV or face of the surf board.

Let the wax dry to a dull, white haze.

Buff the dry wax off the fiberglass surface with a soft rag.

Marine grade fiberglass wax helps to hide the repairs so they blend seamlessly into the surface.

Fiberglass You Can Fix


Shower Walls

Outdoor Furniture

Window Parts





Sky Light Parts

Fiberglass cracks that are less than ¼ inch wide.

What You Can’t Fix

Cracks that permeate through the side of a boat, canoe, kayak or are wider than ¼ inch. These repairs can be repaired but require more aggressive measures and may require help of a professional to repair.


If you can’t match the epoxy repair putty color to the existing color, choose white repair putty. Apply an epoxy based paint made specifically for fiberglass or non porous surfaces.

Repair all cracks as soon as the damage occurs or as soon as you notice the damage. The longer you leave the cracks, the worse they will become.

If you have any dull areas of fiberglass, you can repair the surface to make it look shiny and new.

Rub a rag through automotive polishing compound.

Apply the polishing compound to the fiberglass surface making small circles, continue to rub out the dull areas or scratched areas until the surface is smooth.

Wipe the excess polishing compound off the fiberglass with a clean rag.

Apply a coat of marine grade wax over the entire wall, surfboard side of the boat, tub and shower surround or whatever fiberglass surface you are repairing.

Let the wax dry according to the wax manufacturer’s directions

Buff the surface clean and shiny with a chamois cloth.

Apply a marine grade fiberglass wax to fiberglass surfaces several times a year to maintain a protective coating on the surface. If the items are exposed to weather conditioners, apply wax more often than those items that are used indoors.