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.

Get Your Fill

When you restart the repair, whether after a few days or a few months, vigorously wash the blisters a final time using a stiff brush and as much water pressure as you can bring (Figure 3). This is to flush away any contaminants that drying has brought to the surface. It will not raise the water content of the laminate.

The only resin choice for individual blister repair is epoxy resin, and not the kind hang-in on the pegs at your local hardware store. You need a high-performance epoxy such as West System 105. If the ambient temperature allows, use the regular hardener (for example, West 205), which delivers slightly better moisture exclusion than a slower hardener. You'll be filling the blisters in two or perhaps three steps, and it's essential that each happens before the previous application reaches full cure so that they join together chemically.

Illustration of painting the inside of each cavity with epoxy resin.

Paint the inside of each cavity with epoxy resin.

You should initially attempt to fill just a few blisters to get a feel for how long each step requires and how much curing time to allow.

The first step is to mix up a small amount of epoxy without any additive. Using a small disposable brush, paint the interior of each blister cavity with the epoxy (Figure 4). It is this coat that forms the bond with the existing laminate, so you want the epoxy to completely saturate fibers and penetrate pores and crevices. If you go immediately to the next step, your filler is going to skid on the wet epoxy and slide out of the cavity. To avoid this, wait up to an hour to allow the wet-out coat to partially cure. When it has stiffened but remains tacky, the blister is ready for filling.

Illustration of replacing fiberglass

Replace fiberglass as needed.

Illustration of saturating a cloth with epoxy resin

Then, saturate the cloth with epoxy resin.

Thickened epoxy resin alone is suitable to replace the gelcoat and the first layer or two of the underlying laminate, but where a blister involves deeper layers, it's good practice to replace the fiberglass reinforcement that you ground away. It's not necessary or even particularly desirable to try to match the original lay-up schedule when repairing blisters. Binders in fiberglass mat often make it incompatible with epoxy, so the best choice is six- to 10-ounce fiberglass cloth. Cut the cloth into discs slightly larger than the flat part of the depressions and press them into the tacky wet-out coat. Saturate the cloth with epoxy resin, thickening the resin slightly with colloidal silica if you experience difficulty with the resin draining out of the weave. Add and saturate as many layers of cloth as necessary to restore the laminate to its original thickness (Figure 5). Allow this lay-up to cure for around 30 minutes before filling the remainder of the cavity with silica-thickened epoxy.

Illustration of troweling the filler into the cavity and smooth it

Trowel the filler into the cavity,
smooth it, and then fair it.

For this you want to again mix up a small amount of epoxy resin, and then stir in colloidal silica (West 406, Aerosil/Cabosil, for instance) to thicken the resin to the consistency of creamy peanut butter. The filler in this case is also the barrier coat, and colloidal silica will not compromise the resin's resistance to moisture penetration. Use a flexible plastic spreader to trowel the filler into the cavity and smooth and fair it (Figure 6).

Learn to curve the spreader with thumb pressure to copy any curvature of the hull, and take extra time to try to match the surface of the fill perfectly to the hull. As colloidal silica makes the cured filler resist sanding, extra time here pays generous dividends. Epoxy does not shrink, so there's no need to overfill. A topcoat of unthickened resin after partial cure of the filler can improve the fairness of the repair.

The Final Hurdle

That's it! Let the epoxy cure overnight, then use a Scotch-Brite pad and plenty of water to remove the waxy amine coating. Follow that with a bit of block sanding with 100-grit paper to fair and scuff the surface of the epoxy, and you're ready for bottom paint. Fixing existing blisters is the first step toward figuring out whether a more extensive and vastly more expensive repair will be required. At the very least, repairing individual blisters puts off a more costly response for another year. At best, it avoids that expense altogether. Repairing a few new blisters every year can be a low-cost, long-term strategy.End of story marker

Sailor, author, and DIY guru Don Casey has been a longtime contributor to BoatUS and Ask the Experts.

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