Repairing Blistering On Your Boat: A Comprehensive Guide

Blistering on boats is a common issue, affecting around 25% of boats, and can be caused by long-term immersion of the hull in water, especially in warmer climates. While not all blisters will cause significant problems, if left untreated, they can lead to structural damage. To repair boat blisters, also known as chickenpox, boat owners can use an epoxy resin, a filler, and a hardener to fill in the affected areas. This process requires safety gear, including goggles, as the pressure inside blisters can be dangerous. After draining and sanding the blistered area, it is important to clean and dry the surface thoroughly before applying the epoxy mixture. For deeper blisters, additional steps may be required, such as cutting and layering fiberglass cloth to replace the damaged area. While it is possible to repair boat blisters, prevention is key, and boat owners should consider using a floating boat lift system to reduce the time their boat spends in the water.

Drain and sand shallow blisters

To repair shallow blisters on your boat, you'll first need to drain them. Put on safety gear, especially goggles, as the pressure in a blister is much higher than you might think and the fluid inside is acidic. Use a tool like a chisel or a screwdriver to pop through the gelcoat and release the fluid.

Once drained, sand the area down using a 36-grit sanding disc. Only sand as deep as is necessary to reach fresh, clean laminate. Be careful not to sand too aggressively. After sanding, wash the area with a solution of TSP and hot water—a quarter cup of TSP to a gallon of water is recommended.

Let the surface dry completely before proceeding. Depending on your vessel, this could take as little as 48 hours or up to a month. To test if the surface is dry, tape a square of plastic over the sanded area, sealing it with tape. If any moisture develops on the plastic, the surface is not ready.

Once the surface is dry, scrub the hull surface with acetone on a rag. Then, paint on a layer of unmixed epoxy and let it sit for about half an hour. After that, mix your epoxy with a silica filler and fill the hull depression. It should be about five times thicker than a paint layer. Use a squeegee to smooth and fair the filler.

Prepare the area with acetone and epoxy

To prepare the area with acetone and epoxy, you will need a quart of acetone, a few acid brushes, and a clean rag.

Just before filling the blisters, scrub each depression briskly with the rag dampened with acetone. Mix a small quantity of epoxy (one pump) and paint this unthickened resin into the cavity. Use an acid brush to apply the epoxy and give this application 20 to 30 minutes to begin to kick.

For shallow blisters, prepare a small amount of fresh epoxy (one or two pumps) and thicken it to a peanut butter consistency with colloidal silica. Fill the depression completely with this mix, using a squeegee to compress and fair the filler.

Silica-thickened epoxy is difficult to sand, so take extra time to fair the epoxy as well as possible while it is wet.

Mix epoxy and filler to fill the depression

To repair boat hull blisters, you'll need to mix epoxy resin with a filler to fill in the depression left by the blister. Here's a step-by-step guide on how to do this:

• Prepare the work area by putting on safety gear, including goggles, to protect yourself from the acidic fluid inside the blisters.
• Drain the fluid from the blisters by popping them with a tool like a chisel or a screwdriver.
• Sand the area around the blister with a 36-grit sanding disc. Ensure you only sand down to the fresh, clean laminate.
• Wash the area with a mixture of trisodium phosphate (TSP) and hot water (a quarter cup of TSP per gallon of water).
• Let the surface dry completely. This could take 48 hours or even up to a month, depending on your vessel.
• Tape a square of plastic over the sanded area and seal it with tape. Wait 24 hours to check for any moisture build-up. If the area is dry, you can proceed.
• Scrub the hull surface with acetone on a rag to prepare it for the epoxy.
• Paint a layer of unmixed epoxy on the area and let it sit for about 30 minutes.
• Now, it's time to mix your epoxy and filler. For the filler, use colloidal silica, which is not absorbent and can effectively fill the blisters. Do not use talc or any absorbent materials, as they will make the problem worse. Mix the epoxy and silica filler to a peanut butter-like consistency.
• Fill the hull depression with the epoxy and filler mixture. Use a squeegee to smooth it out and shape it to match the boat's contours.
• Allow the filler to cure.
• Once cured, you can paint over the repaired area with antifouling paint. Do not apply gel coat over the epoxy, as this can lead to the recurrence of blisters.

Remember to always work in a well-ventilated area when working with epoxy and wear the appropriate safety gear, including gloves and a respirator, in addition to eye protection.

Repair deep blisters by replacing the glass fabric

To repair deep blisters, you will need to replace the damaged glass fabric. Here is a step-by-step guide:

• Prepare the work area: Put on safety gear, including eye protection and gloves. Blisters contain acidic liquid that can be dangerous if it comes into contact with your skin or eyes.
• Open the blisters: Use a chisel, screwdriver, or a utility knife to pop the blisters and let the liquid drain out. Be cautious as the liquid is under pressure and may gush out.
• Inspect the damage: Tap around the edges of the open blister with a screwdriver or chisel. A dull sound indicates delamination, meaning the damage is more extensive than initially thought.
• Grind the blister: Use a disk grinder with a 36-grit sanding disk to grind the open blister into a shallow depression. Grind until you reach fresh, undamaged laminate beneath the gelcoat. The depression should be wide and shallow, approximately 20 times wider than its depth.
• Clean and dry the area: Flush the open blister with water to remove any loose particles. Then, scrub the area with a solution of hot water and trisodium phosphate (TSP) to remove any remaining chemicals. Rinse thoroughly and allow the blister to dry completely. This may take at least 48 hours or longer, depending on the severity of the blistering.
• Prepare the epoxy resin: Mix a small batch of epoxy resin with a filler such as colloidal silica to achieve a peanut butter-like consistency.
• Cut the fiberglass cloth: Cut several pieces of fiberglass cloth with increasing diameters. The largest piece should match the size of the depression, and each subsequent piece should be slightly smaller. Ensure you only use fiberglass cloth and avoid chopped strand mat (CSM) unless specifically intended for epoxy use.
• Apply the epoxy and fiberglass: Wet the bottom of the cavity with epoxy and lay the largest piece of fiberglass cloth into it. Saturate the cloth with epoxy resin until it becomes transparent. Repeat this process with the remaining pieces of cloth, placing them in order from largest to smallest. Ensure each layer is well-saturated and compressed together.
• Allow the repair to cure: Depending on the depth of the blister, you may need to pause after every 4-5 layers of cloth to allow the epoxy to cure and the heat to dissipate.
• Finish the repair: Once the epoxy is cured, scrub the repair with water and an abrasive pad to remove any residue. Sand the repair area with a sanding block to smooth out any irregularities. You can then paint over the repair with antifouling paint. Do not apply gelcoat over the epoxy repair.

Remember to practice good boat maintenance and regularly inspect your vessel for blisters. While most blisters are cosmetic issues, leaving them untreated can lead to more severe problems such as boat pox, which requires specialist attention.

Apply epoxy and filler to the patched area

To apply epoxy and filler to the patched area, start by scrubbing the area with a clean rag dampened with acetone. Then, mix a small amount of epoxy and paint this unthickened resin into the cavity. Wait around 20 to 30 minutes for it to kick.

Next, prepare a small amount of fresh epoxy and thicken it with colloidal silica to a peanut butter-like consistency. Fill the depression with this mixture, using a squeegee to compress and fair the filler. Take your time to fair the epoxy while it is still wet, as silica-thickened epoxy is difficult to sand.

If you are dealing with deep blisters, you will need to replace the damaged glass fabric. Cut a disk of fiberglass cloth to match the diameter of the depression, and then cut several more disks, each slightly smaller than the last. Wet the bottom of the cavity with epoxy and lay the largest disk of cloth. Saturate the cloth with resin until it is transparent, and then repeat this process with the remaining disks, placing them in descending order of size. Continue adding disks and saturating them with resin until the repair is even with the surrounding surface. Remember to pause after every 4 to 5 layers to allow the epoxy to kick and the heat to dissipate.

Once the filler has kicked, paint the repair and an inch or two beyond with at least two coats of unthickened epoxy, letting each coat kick before applying the next. Allow the repair to cure for 24 hours, and then scrub it with water and an abrasive pad to remove any waxy residue. Finally, fair the repair with a sanding block, and your boat's patched area will be as good as new!

Frequently asked questions