Gelcoat Blister Repair

By Don Casey
Published: June/July 2013

The discovery of hull blisters elicits a mental "oh no!" from most boat owners. But in many cases, the solution is simple and inexpensive.

Find And Grind

Illustration of breaking open a blister

Break open each blister.

Blisters can deflate in as little as an hour after a boat is lifted from the water, so it's best to examine the bottom immediately to avoid drawing a false conclusion. Even after shrink-in, blisters will normally be higher than the surrounding surface. If you're not there for the haul-out, a useful trick later is to scuff the bottom lightly with a coarse sanding block to locate and mark high spots.

There's nothing complicated about blister repair. It starts with breaking open every blister. Pressurized acidic liquid often sprays out, so wear eye protection and use the corner of a sharp chisel at arm's length (Figure 1). Damage can extend beyond the dome, so reverse the chisel and use the handle as a plastic mallet to tap the laminate all around each blister or blistered area. Solid laminate will sound sharp, voids will sound dull. Mark the perimeter of suspect areas. After all blisters are open, wash the hull to flush away acidic trickles.

Illustration of grinding away the broken dome of the blister

Grind away the broken dome of the blister.

Once the hull dries, grind away the remains of the broken dome with a two-inch sanding disc (Roloc) or some other sanding or grinding bit in a drill or rotary tool (Figure 2). Chamfer the perimeter of every blister to transform it into a shallow dimple. Typically you'll be removing only gelcoat, but if you discover damaged laminate, you'll need to grind this away until you reach healthy laminate. Wetting laminate with a trigger sprayer can help you to differentiate. Damaged or dry laminate will show white fibers while healthy laminate will appear dark and translucent with the fibers not evident.

Whether a depression ends up spoon-, saucer-, or plate-size will be determined by how far out you have to grind to encounter a solid bond between gelcoat and laminate. Anywhere the exposed edge exhibits separation between gelcoat and laminate, keep expanding the perimeter. Open any separate voids your sounding discovered and treat these the same way as a blister, dishing them into shallow depressions.

Know Your Enemies

Illustration of scrubing the surface clean

Scrub until squeaky clean with a stiff brush.

Loose debris is one enemy, so scrub the prepared blisters squeaky clean with a stiff brush and plenty of water (Figure 3). Another enemy is moisture. If you've waited to do the job after hauling the boat for the off-season, this is where to stop. By waiting to fill the blisters right before you launch next season, you take maximum advantage of the potential for the exposed laminate to dry out. But if the boat isn't going to stay out of the water, you'll need to dry the laminate before filling the blisters. Grinding will remove much of the wettest material. Blot-wiping each cavity with isopropyl alcohol-soaked paper towels will draw out much of the remaining moisture. Completely wet the laminate with rub-big alcohol, then wipe away contaminants it brings up. Repeat with soaked, fresh paper towels until they come away clean.

Normally this will leave the cavity both clean enough and dry enough to fill, but you can check for moisture content by taping squares cut from zipper freezer bags over a few of the blisters. Seal the entire perimeter of the clear plastic squares to the hull with duct tape, and then take a three-day break. If the weather is warm enough for a sensible person to be working on a boat and there's still moisture in the laminate, it will con-dense onto the inside of the clear plastic. In the likely event that the plastic is dry, you're good to go. If it's fully coated with condensation, your repair will benefit from giving the open blisters at least a couple of low-humidity weeks to dry out.

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