How to Repair Boat Fiberglass

Keep your boat sailing with a fiberglass patch.

If you have made a financial investment in a boat, then you know there are few things scarier than watching a hole sink your purchase. A hole does needn't be the end of your fiberglass boat, however. Fiberglass is easily repaired. With the proper preparation, you can create a patch that will be indistinguishable from the original surface.

Things You'll Need

• Screwdriver
• Saw
• Dewaxing solvent
• Scrub brush
• Grinder
• Paint brushes
• Acetone
• Rag
• Paste wax
• Stiff plastic sheet
• Scissors
• Duct tape
• Fiberglass mat an cloth
• Resin and catalyst
• Gelcoat and catalyst (optional)
• Disposable buckets
• Resin roller or squeegee
• Palm sander and various grits of sandpaper
• Paint (Optional)
• Wax (Optional)
• Polish (Optional)

Instructions

Preparing the Boat

1. Pull the boat out of the water and allow it to dry if the damage is below the waterline.

2. Tap the area around the damage with a screwdriver handle to determine its extent. Solid areas make a sharp sound, while damaged areas sound dull.

3. Cut away all damaged fiberglass. Cutting a circular or oval hole (as opposed to an irregular hole precisely following the damage) will best preserve the hull's strength.

4. Wash the inner hull around the hole with a de-waxing solvent to ensure a clean working surface. From this point on (unless specified) you will be working on the inside of the boat.

5. Grind the area around the damage. The ground area should extend a few inches beyond the hole, and will allow you to build up a fiberglass patch without creating a noticeable lump on the inner hull.

6. Brush away any dust and wash the area with acetone.

7. Apply a heavy ring of paste wax to the outer hull around the hole to prevent any resin drips from sticking.

8. Cut a sheet of stiff plastic a foot larger than the hole. Wax it and tape it securely in place on the outer hull over the hole. This will form a backing for the patch and make it much easier to obtain a smooth finish.

Repairing the Hole

1. Cut several layers of fiberglass mat and cloth 1/2 inch larger than the hole. The number of pieces will depend on the thickness of the hull, but generally you will want one layer for every 1/32 inch.

2. Mix a small batch of gelcoat (thick resin) and catalyst in a disposable bucket, following the instructions for the product that you are using. For repairs above the waterline you can use standard polyester resin. For repairs below the waterline, use epoxy and skip the gelcoat steps.

3. Working from the inside of the boat, brush a layer of catalyzed gelcoat onto the waxed surface of the plastic sheet. It should be about 1/32 inch thick and will provide a (mostly) smooth outer surface. Allow the gelcoat to cure until it is dry and no longer sticky.

4. Mix a batch of resin and catalyst in a fresh bucket.

5. Brush a coat of catalyzed resin onto the gelcoat.

6. Apply a piece of fiberglass mat into the resin. Mat provides a smoother surface area, which is why you will use it for the first (outer) few layers.

7. Tap more resin into the fiberglass to fully saturate it. Taping the resin instead of brushing it helps to eliminate trapped air bubbles that weaken the fiberglass.

8. Add another layer of fiberglass mat and resin, then alternate between cloth and mat until you have filled the thickness of the hole.

9. Finish with another two layers of mat, cut slightly larger than the rest of the patch to ensure a good blend. Use a resin roller or squeegee to get the patch as smooth as possible.

10. Allow the fiberglass to cure fully.

11. Sand the inner surface smooth if people will come into contact with it. If not, skip this step.

12. Remove the plastic sheet from the outer hull and fill in any imperfections with fresh gelcoat.

13. Allow any new gelcoat to cure, and then sand the entire area smooth. If needed, paint, wax and polish the area.

Tips & Warnings

• Get the best ventilation possible when working with fiberglass. This can be tricky when working in an enclosed space, but you can use fans to create some airflow. You don't want to be breathing nothing but trapped resin fumes.

• Always wear gloves, goggles and a dust mask when working with fiberglass. This includes grinding, sanding and working with fresh resin.

References

• Boat US: Fiberglass Repair
• Serious Explorations: A step by step guide to creating with fiberglass