Fiberglass Repair Kit: How to Patch Fiberglass
How a fiberglass repair kit patches cracks, chips and holes: resin and cloth kits, two-part fillers, mix ratios, hardening times and which kit fits a tub, a boat or a car.
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A fiberglass repair kit patches a cracked tub, a chipped boat hull or a holed car panel with resin, cloth or filler that hardens into a solid, water-resistant patch. Pick the kit by the job: a two-part filler for a tub chip, a resin and cloth kit for a structural crack, and a marine gel coat kit for a boat above the waterline. This guide explains what each kind of kit holds, how to mix and apply it, and how long a repair takes to harden.
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Below: the kinds of kit in the listings, what is in the box, the step by step repair, hardening times from the listings, and when a repair is beyond a kit.
Fiberglass repair kit: the kinds in the listings
| Kit type | What it holds | Suits | Time stated in the listing |
|---|---|---|---|
| Two-part tub filler | Mixed paste, tints | Tub chips, cracks, small holes | 12 hours (FENFANDAS) |
| Marine gel coat filler | Epoxy filler paste | Boats above the waterline | 24 hours (Globalcom); more than 24 hours (Mallbaola) |
| Resin and cloth kit | Resin, hardener, cloth | Structural cracks, cars, boats | 24 hours (YOTEBLE); sandable in under 2 hours (Bondo) |
| Fiberglass putty | Short-strand putty | Cracks up to ½ inch | Dries in 30 minutes (J-B Weld) |
The kit type sets the repair. A paste filler fills a chip and sands smooth, while a resin and cloth kit rebuilds strength across a crack with layers of cloth. A marine gel coat kit is made to blend with a boat finish, and a tub filler is made to match a bath finish. Buying the wrong kind means a patch that looks wrong or does not hold.
What is in the box
A kit is more than resin. The Bondo 00420 kit includes fiberglass resin, liquid hardener, a plastic spreader, fiberglass cloth, a mixing stick and a mixing tray. One Mshunya listing states 200g of resin, 100g of hardener and 300g of fiberglass cloth, with a 60ml measuring cup, 2 stirring sticks, an application brush, protective gloves, 3 sheets of sandpaper and an illustrated manual. The YOTEBLE kit includes resin, liquid hardener, fiberglass cloth, a mixing stick, a brush and a measuring cup, in 120g plus 60g sizes. Tub kits are simpler: the FENFANDAS kit is a two-part paste, and the Ankita kit adds tints to blend with white, aged white, off-white, almond, beige, bone, biscuit and ivory finishes.
Kit sizes run from small to large. The Bondo 00420 kit is 0.45 Pint and the Bondo 00422 kit is 0.9 Quart, with the same contents otherwise. The Mshunya kit in one listing holds 200g of resin and 100g of hardener, and the YOTEBLE kit holds 120g plus 60g. The listings do not state a coverage area, so match the kit size to the damage and keep a spare on hand for a second coat.
How to patch fiberglass, step by step
- Clean and dry the area. The Globalcom listing says to clean the repair area thoroughly and keep it dry. Resin bonds poorly to a greasy or damp surface.
- Mask the surround. Cover the area around the damage with tape so the paste or resin does not spread onto the good finish.
- Mix parts A and B in the ratio the kit states. FENFANDAS, Globalcom and Mallbaola kits mix 1:1, while Mshunya and YOTEBLE kits mix 2:1. Mix until the color is uniform, and mix only as much as the job needs.
- Apply and smooth. Spread the paste or resin over the damage, smooth it with a scraper or spreader, and lay cloth over a structural crack before the resin sets. The Mshunya listing says to apply multiple thin coats instead of one thick coat, and to wear safety goggles on vertical surfaces.
- Wait for it to harden. The FENFANDAS kit becomes hard after 12 hours, Globalcom and YOTEBLE kits harden in 24 hours, and the Mallbaola kit needs more than 24 hours. The J-B Weld putty dries in 30 minutes and the Bondo kits are sandable in under 2 hours. Hardening time varies with temperature and humidity, as the Mshunya listing notes.
- Sand it smooth. Sand the hardened patch with sandpaper until it is smooth, then paint or finish it to match. The J-B Weld putty is sandable and paintable, and the Bondo kits are sandable in under 2 hours.
Resin and hardener can irritate skin, and some give off fumes while they harden. Work with ventilation, wear the gloves the kit includes and goggles, and keep resin off skin and out of eyes. If resin A has frozen in cold weather, the Mshunya listings say to thaw it in warm water, 10 minutes in 60°C water or 7 minutes in 80 to 100°C water, and shake it before use.
Use rated jack stands on solid ground, wear eye protection with power and air tools, keep fuel-burning equipment out of a closed garage, and follow the manual for every tool. Read our Safety Disclaimer.
Which kit fits which job
For a tub or shower, use a filler made for bath finishes. The FENFANDAS kit lists fiberglass, acrylic, ceramic, enamel and cast iron tubs, plus porcelain sinks, ceramic tile, toilets and cultured marble. The Ankita kit adds tints for white, aged white, off-white, almond, beige, bone, biscuit and ivory, so the patch does not show as a bright white spot on an older tub.
For a boat, use a marine kit. The Globalcom kit lists jet skis, canoes, kayaks, sailboats, speedboats, yachts, dinghies, surfboards and decks, and the Mallbaola kit states it is only for boat parts above the waterline. The YOTEBLE kit lists the same kind of watercraft and surfboards. A marine kit is made to stand up to sun and water, and the Globalcom listing states UV resistance and a waterproof finish.
For a car panel or a structural crack, use resin and cloth or a reinforced putty. The J-B Weld putty repairs cracks up to ½ inch on fiberglass, composites, metal and concrete, and the maker claims 2x the strength of body filler. The Mshunya kit is made for structural-level repairs and bonds carbon fiber, fiberglass, metal, wood and plastic. The Bondo kits list automobiles, boats, concrete, wood, fiberglass and metal.
When a kit is not enough
A kit covers localized damage, not a rebuild. A crack past the ½ inch the J-B Weld putty states is beyond that putty, and a hull below the waterline is beyond the Mallbaola gel coat kit, which states above the waterline only. A repair that flexes, leaks or keeps cracking after a patch has been tried is a job for a pro shop, not a second coat from the same kit.
FAQ: garage tools
How long does a fiberglass repair take to harden?
It depends on the kit. The FENFANDAS kit becomes hard after 12 hours, Globalcom and YOTEBLE kits harden in 24 hours, and the Mallbaola kit needs more than 24 hours. The J-B Weld putty dries in 30 minutes and the Bondo kits are sandable in under 2 hours. Hardening time varies with temperature and humidity, as the Mshunya listing notes.
What mix ratio does a fiberglass repair kit use?
FENFANDAS, Globalcom and Mallbaola kits mix parts A and B in a 1:1 ratio, while Mshunya and YOTEBLE kits mix 2:1. Follow the ratio on the kit in hand, because the wrong ratio leaves a patch that stays soft or never fully hardens.
Can I use a fiberglass repair kit on a bathtub?
Yes. The FENFANDAS kit lists fiberglass, acrylic, ceramic, enamel and cast iron tubs, plus porcelain sinks, ceramic tile, toilets and cultured marble. The Ankita kit adds tints for white, aged white, off-white, almond, beige, bone, biscuit and ivory finishes.
Can I repair a boat below the waterline with a gel coat kit?
No. The Mallbaola listing states its gel coat repair kit is only for use on boat parts above the waterline. A below-waterline repair needs a product rated for immersion, which these listings do not state.
How big a crack can a fiberglass filler repair?
The J-B Weld putty states repairs to cracks up to ½ inch on fiberglass, composites, metal and concrete. Larger damage needs cloth and resin laid in layers. A repair that keeps cracking after a patch has been tried needs a pro shop.
Do I need fiberglass cloth, or is filler enough?
Filler is enough for chips, small holes and surface cracks. A structural crack needs cloth and resin to rebuild strength, and the Bondo kits include fiberglass cloth while one Mshunya listing states 300g of fiberglass cloth. Filler alone does not add the strength that cloth gives.
Related: the garage upgrades hub, car detailing for finishing a repair, and the engine and shop equipment hub. Also see our safety disclaimer.
